Change `ActionDispatch::Response#content_type` returning Content-Type header as it is

Since #35709, `Response#conten_type` returns only MIME type correctly.
It is a documented behavior that this method only returns MIME type, so
this change seems appropriate.
https://github.com/rails/rails/blob/39de7fac0507070e3c5f8b33fbad6fced84d97ed/actionpack/lib/action_dispatch/http/response.rb#L245-L249

But unfortunately, some users expect this method to return all
Content-Type that does not contain charset. This seems to be breaking
changes.

We can change this behavior with the deprecate cycle.
But, in that case, a method needs that include Content-Type with
additional parameters. And that method name is probably the
`content_type` seems to properly.

So I changed the new behavior to more appropriate `media_type` method.
And `Response#content_type` changed (as the method name) to return Content-Type
header as it is.

Fixes #35709.

[Rafael Mendonça França & Yuuji Yaginuma ]

23 files changed, 171 insertions, 120 deletions

diff --git a/actionpack/lib/action_controller/metal.rb b/actionpack/lib/action_controller/metal.rb
index b9088e6d86..9ca0bbb9db 100644
--- a/actionpack/lib/action_controller/metal.rb
+++ b/actionpack/lib/action_controller/metal.rb
@@ -148,7 +148,7 @@ module ActionController
     attr_internal :response, :request
     delegate :session, to: "@_request"
     delegate :headers, :status=, :location=, :content_type=,
-             :status, :location, :content_type, to: "@_response"
+             :status, :location, :content_type, :media_type, to: "@_response"
 
     def initialize
       @_request = nil
diff --git a/actionpack/lib/action_controller/metal/mime_responds.rb b/actionpack/lib/action_controller/metal/mime_responds.rb
index bf5e7a433f..5c6f7fe396 100644
--- a/actionpack/lib/action_controller/metal/mime_responds.rb
+++ b/actionpack/lib/action_controller/metal/mime_responds.rb
@@ -205,7 +205,7 @@ module ActionController #:nodoc:
       yield collector if block_given?
 
       if format = collector.negotiate_format(request)
-        if content_type && content_type != format
+        if media_type && media_type != format
           raise ActionController::RespondToMismatchError
         end
         _process_format(format)
diff --git a/actionpack/lib/action_controller/metal/renderers.rb b/actionpack/lib/action_controller/metal/renderers.rb
index b81d3ef539..a251c29d23 100644
--- a/actionpack/lib/action_controller/metal/renderers.rb
+++ b/actionpack/lib/action_controller/metal/renderers.rb
@@ -157,7 +157,7 @@ module ActionController
       json = json.to_json(options) unless json.kind_of?(String)
 
       if options[:callback].present?
-        if content_type.nil? || content_type == Mime[:json]
+        if media_type.nil? || media_type == Mime[:json]
           self.content_type = Mime[:js]
         end
 
diff --git a/actionpack/lib/action_controller/metal/rendering.rb b/actionpack/lib/action_controller/metal/rendering.rb
index 7d0a944381..7f7c736965 100644
--- a/actionpack/lib/action_controller/metal/rendering.rb
+++ b/actionpack/lib/action_controller/metal/rendering.rb
@@ -73,7 +73,7 @@ module ActionController
       end
 
       def _set_rendered_content_type(format)
-        if format && !response.content_type
+        if format && !response.media_type
           self.content_type = format.to_s
         end
       end
diff --git a/actionpack/lib/action_dispatch/http/response.rb b/actionpack/lib/action_dispatch/http/response.rb
index 69798f99e0..61e3a870ab 100644
--- a/actionpack/lib/action_dispatch/http/response.rb
+++ b/actionpack/lib/action_dispatch/http/response.rb
@@ -243,8 +243,13 @@ module ActionDispatch # :nodoc:
     end
 
     # Content type of response.
-    # It returns just MIME type and does NOT contain charset part.
     def content_type
+      type = super
+      type&.empty? ? nil : type
+    end
+
+    # Media type of response.
+    def media_type
       parsed_content_type_header.mime_type
     end
 
@@ -458,7 +463,7 @@ module ActionDispatch # :nodoc:
     end
 
     def assign_default_content_type_and_charset!
-      return if content_type
+      return if media_type
 
       ct = parsed_content_type_header
       set_content_type(ct.mime_type || Mime[:html].to_s,
diff --git a/actionpack/lib/action_dispatch/testing/assertions.rb b/actionpack/lib/action_dispatch/testing/assertions.rb
index 08c2969685..dcaf914ac9 100644
--- a/actionpack/lib/action_dispatch/testing/assertions.rb
+++ b/actionpack/lib/action_dispatch/testing/assertions.rb
@@ -14,7 +14,7 @@ module ActionDispatch
     include Rails::Dom::Testing::Assertions
 
     def html_document
-      @html_document ||= if @response.content_type.to_s.end_with?("xml")
+      @html_document ||= if @response.media_type.to_s.end_with?("xml")
         Nokogiri::XML::Document.parse(@response.body)
       else
         Nokogiri::HTML::Document.parse(@response.body)
diff --git a/actionpack/lib/action_dispatch/testing/test_response.rb b/actionpack/lib/action_dispatch/testing/test_response.rb
index 6f7c86fdcf..f1dd4099c5 100644
--- a/actionpack/lib/action_dispatch/testing/test_response.rb
+++ b/actionpack/lib/action_dispatch/testing/test_response.rb
@@ -19,7 +19,7 @@ module ActionDispatch
     end
 
     def response_parser
-      @response_parser ||= RequestEncoder.parser(content_type)
+      @response_parser ||= RequestEncoder.parser(media_type)
     end
   end
 end
diff --git a/actionpack/test/controller/content_type_test.rb b/actionpack/test/controller/content_type_test.rb
index 636b025f2c..aeb0d07195 100644
--- a/actionpack/test/controller/content_type_test.rb
+++ b/actionpack/test/controller/content_type_test.rb
@@ -66,68 +66,68 @@ class ContentTypeTest < ActionController::TestCase
   def test_render_defaults
     get :render_defaults
     assert_equal "utf-8", @response.charset
-    assert_equal Mime[:text], @response.content_type
+    assert_equal Mime[:text], @response.media_type
   end
 
   def test_render_changed_charset_default
     with_default_charset "utf-16" do
       get :render_defaults
       assert_equal "utf-16", @response.charset
-      assert_equal Mime[:text], @response.content_type
+      assert_equal Mime[:text], @response.media_type
     end
   end
 
   # :ported:
   def test_content_type_from_body
     get :render_content_type_from_body
-    assert_equal Mime[:rss], @response.content_type
+    assert_equal Mime[:rss], @response.media_type
     assert_equal "utf-8", @response.charset
   end
 
   # :ported:
   def test_content_type_from_render
     get :render_content_type_from_render
-    assert_equal Mime[:rss], @response.content_type
+    assert_equal Mime[:rss], @response.media_type
     assert_equal "utf-8", @response.charset
   end
 
   # :ported:
   def test_charset_from_body
     get :render_charset_from_body
-    assert_equal Mime[:text], @response.content_type
+    assert_equal Mime[:text], @response.media_type
     assert_equal "utf-16", @response.charset
   end
 
   # :ported:
   def test_nil_charset_from_body
     get :render_nil_charset_from_body
-    assert_equal Mime[:text], @response.content_type
+    assert_equal Mime[:text], @response.media_type
     assert_equal "utf-8", @response.charset, @response.headers.inspect
   end
 
   def test_nil_default_for_erb
     with_default_charset nil do
       get :render_default_for_erb
-      assert_equal Mime[:html], @response.content_type
+      assert_equal Mime[:html], @response.media_type
       assert_nil @response.charset, @response.headers.inspect
     end
   end
 
   def test_default_for_erb
     get :render_default_for_erb
-    assert_equal Mime[:html], @response.content_type
+    assert_equal Mime[:html], @response.media_type
     assert_equal "utf-8", @response.charset
   end
 
   def test_default_for_builder
     get :render_default_for_builder
-    assert_equal Mime[:xml], @response.content_type
+    assert_equal Mime[:xml], @response.media_type
     assert_equal "utf-8", @response.charset
   end
 
   def test_change_for_builder
     get :render_change_for_builder
-    assert_equal Mime[:html], @response.content_type
+    assert_equal Mime[:html], @response.media_type
     assert_equal "utf-8", @response.charset
   end
 
@@ -148,22 +148,22 @@ class AcceptBasedContentTypeTest < ActionController::TestCase
   def test_render_default_content_types_for_respond_to
     @request.accept = Mime[:html].to_s
     get :render_default_content_types_for_respond_to
-    assert_equal Mime[:html], @response.content_type
+    assert_equal Mime[:html], @response.media_type
 
     @request.accept = Mime[:js].to_s
     get :render_default_content_types_for_respond_to
-    assert_equal Mime[:js], @response.content_type
+    assert_equal Mime[:js], @response.media_type
   end
 
   def test_render_default_content_types_for_respond_to_with_template
     @request.accept = Mime[:xml].to_s
     get :render_default_content_types_for_respond_to
-    assert_equal Mime[:xml], @response.content_type
+    assert_equal Mime[:xml], @response.media_type
   end
 
   def test_render_default_content_types_for_respond_to_with_overwrite
     @request.accept = Mime[:rss].to_s
     get :render_default_content_types_for_respond_to
-    assert_equal Mime[:xml], @response.content_type
+    assert_equal Mime[:xml], @response.media_type
   end
 end
diff --git a/actionpack/test/controller/integration_test.rb b/actionpack/test/controller/integration_test.rb
index 4dddd98f9f..cce229b30d 100644
--- a/actionpack/test/controller/integration_test.rb
+++ b/actionpack/test/controller/integration_test.rb
@@ -522,11 +522,11 @@ class IntegrationProcessTest < ActionDispatch::IntegrationTest
     with_test_route_set do
       get "/get", headers: { "Accept" => "application/json" }, xhr: true
       assert_equal "application/json", request.accept
-      assert_equal "application/json", response.content_type
+      assert_equal "application/json", response.media_type
 
       get "/get", headers: { "HTTP_ACCEPT" => "application/json" }, xhr: true
       assert_equal "application/json", request.accept
-      assert_equal "application/json", response.content_type
+      assert_equal "application/json", response.media_type
     end
   end
 
@@ -986,7 +986,7 @@ class IntegrationRequestEncodersTest < ActionDispatch::IntegrationTest
   def test_encoding_as_json
     post_to_foos as: :json do
       assert_response :success
-      assert_equal "application/json", request.content_type
+      assert_equal "application/json", request.media_type
       assert_equal "application/json", request.accepts.first.to_s
       assert_equal :json, request.format.ref
       assert_equal({ "foo" => "fighters" }, request.request_parameters)
@@ -1025,7 +1025,7 @@ class IntegrationRequestEncodersTest < ActionDispatch::IntegrationTest
     post_to_foos as: :wibble do
       assert_response :success
       assert_equal "/foos_wibble", request.path
-      assert_equal "text/wibble", request.content_type
+      assert_equal "text/wibble", request.media_type
       assert_equal "text/wibble", request.accepts.first.to_s
       assert_equal :wibble, request.format.ref
       assert_equal Hash.new, request.request_parameters # Unregistered MIME Type can't be parsed.
diff --git a/actionpack/test/controller/localized_templates_test.rb b/actionpack/test/controller/localized_templates_test.rb
index d84a76fb46..5c5cef66d5 100644
--- a/actionpack/test/controller/localized_templates_test.rb
+++ b/actionpack/test/controller/localized_templates_test.rb
@@ -43,6 +43,6 @@ class LocalizedTemplatesTest < ActionController::TestCase
     I18n.locale = :it
     get :hello_world
     assert_equal "Ciao Mondo", @response.body
-    assert_equal "text/html",  @response.content_type
+    assert_equal "text/html",  @response.media_type
   end
 end
diff --git a/actionpack/test/controller/metal/renderers_test.rb b/actionpack/test/controller/metal/renderers_test.rb
index 5f0d125128..f6558f1354 100644
--- a/actionpack/test/controller/metal/renderers_test.rb
+++ b/actionpack/test/controller/metal/renderers_test.rb
@@ -38,13 +38,13 @@ class RenderersMetalTest < ActionController::TestCase
     get :one
     assert_response :success
     assert_equal({ a: "b" }.to_json, @response.body)
-    assert_equal "application/json", @response.content_type
+    assert_equal "application/json", @response.media_type
   end
 
   def test_render_xml
     get :two
     assert_response :success
     assert_equal(" ", @response.body)
-    assert_equal "text/plain", @response.content_type
+    assert_equal "text/plain", @response.media_type
   end
 end
diff --git a/actionpack/test/controller/mime/respond_to_test.rb b/actionpack/test/controller/mime/respond_to_test.rb
index 2f8f191828..fc16c639fb 100644
--- a/actionpack/test/controller/mime/respond_to_test.rb
+++ b/actionpack/test/controller/mime/respond_to_test.rb
@@ -423,12 +423,12 @@ class RespondToControllerTest < ActionController::TestCase
   def test_using_defaults
     @request.accept = "*/*"
     get :using_defaults
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "Hello world!", @response.body
 
     @request.accept = "application/xml"
     get :using_defaults
-    assert_equal "application/xml", @response.content_type
+    assert_equal "application/xml", @response.media_type
     assert_equal "<p>Hello world!</p>\n", @response.body
   end
 
@@ -449,12 +449,12 @@ class RespondToControllerTest < ActionController::TestCase
   def test_using_defaults_with_type_list
     @request.accept = "*/*"
     get :using_defaults_with_type_list
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "Hello world!", @response.body
 
     @request.accept = "application/xml"
     get :using_defaults_with_type_list
-    assert_equal "application/xml", @response.content_type
+    assert_equal "application/xml", @response.media_type
     assert_equal "<p>Hello world!</p>\n", @response.body
   end
 
@@ -468,7 +468,7 @@ class RespondToControllerTest < ActionController::TestCase
   def test_using_non_conflicting_nested_js_then_js
     @request.accept = "*/*"
     get :using_non_conflicting_nested_js_then_js
-    assert_equal "text/javascript", @response.content_type
+    assert_equal "text/javascript", @response.media_type
     assert_equal "JS", @response.body
   end
 
@@ -499,12 +499,12 @@ class RespondToControllerTest < ActionController::TestCase
   def test_custom_types
     @request.accept = "application/fancy-xml"
     get :custom_type_handling
-    assert_equal "application/fancy-xml", @response.content_type
+    assert_equal "application/fancy-xml", @response.media_type
     assert_equal "Fancy XML", @response.body
 
     @request.accept = "text/html"
     get :custom_type_handling
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "HTML", @response.body
   end
 
@@ -595,7 +595,7 @@ class RespondToControllerTest < ActionController::TestCase
     @request.accept = "application/json"
     get :json_with_callback
     assert_equal "/**/alert(JS)", @response.body
-    assert_equal "text/javascript", @response.content_type
+    assert_equal "text/javascript", @response.media_type
   end
 
   def test_xhr
@@ -605,13 +605,13 @@ class RespondToControllerTest < ActionController::TestCase
 
   def test_custom_constant
     get :custom_constant_handling, format: "mobile"
-    assert_equal "text/x-mobile", @response.content_type
+    assert_equal "text/x-mobile", @response.media_type
     assert_equal "Mobile", @response.body
   end
 
   def test_custom_constant_handling_without_block
     get :custom_constant_handling_without_block, format: "mobile"
-    assert_equal "text/x-mobile", @response.content_type
+    assert_equal "text/x-mobile", @response.media_type
     assert_equal "Mobile", @response.body
   end
 
@@ -664,7 +664,7 @@ class RespondToControllerTest < ActionController::TestCase
     assert_equal '<html><div id="html">Hello future from Firefox!</div></html>', @response.body
 
     get :iphone_with_html_response_type, format: "iphone"
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal '<html><div id="iphone">Hello iPhone future from iPhone!</div></html>', @response.body
   end
 
@@ -672,7 +672,7 @@ class RespondToControllerTest < ActionController::TestCase
     @request.accept = "text/iphone"
     get :iphone_with_html_response_type
     assert_equal '<html><div id="iphone">Hello iPhone future from iPhone!</div></html>', @response.body
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_invalid_format
@@ -702,7 +702,7 @@ class RespondToControllerTest < ActionController::TestCase
 
   def test_variant_with_implicit_template_rendering
     get :variant_with_implicit_template_rendering, params: { v: :mobile }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "mobile", @response.body
   end
 
@@ -756,137 +756,137 @@ class RespondToControllerTest < ActionController::TestCase
 
   def test_variant_with_format_and_custom_render
     get :variant_with_format_and_custom_render, params: { v: :phone }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "mobile", @response.body
   end
 
   def test_multiple_variants_for_format
     get :multiple_variants_for_format, params: { v: :tablet }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "tablet", @response.body
   end
 
   def test_no_variant_in_variant_setup
     get :variant_plus_none_for_format
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "none", @response.body
   end
 
   def test_variant_inline_syntax
     get :variant_inline_syntax
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "none", @response.body
 
     get :variant_inline_syntax, params: { v: :phone }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
   end
 
   def test_variant_inline_syntax_with_format
     get :variant_inline_syntax, format: :js
-    assert_equal "text/javascript", @response.content_type
+    assert_equal "text/javascript", @response.media_type
     assert_equal "js", @response.body
   end
 
   def test_variant_inline_syntax_without_block
     get :variant_inline_syntax_without_block, params: { v: :phone }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
   end
 
   def test_variant_any
     get :variant_any, params: { v: :phone }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
 
     get :variant_any, params: { v: :tablet }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "any", @response.body
 
     get :variant_any, params: { v: :phablet }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "any", @response.body
   end
 
   def test_variant_any_any
     get :variant_any_any
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "any", @response.body
 
     get :variant_any_any, params: { v: :phone }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
 
     get :variant_any_any, params: { v: :yolo }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "any", @response.body
   end
 
   def test_variant_inline_any
     get :variant_any, params: { v: :phone }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
 
     get :variant_inline_any, params: { v: :tablet }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "any", @response.body
 
     get :variant_inline_any, params: { v: :phablet }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "any", @response.body
   end
 
   def test_variant_inline_any_any
     get :variant_inline_any_any, params: { v: :phone }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
 
     get :variant_inline_any_any, params: { v: :yolo }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "any", @response.body
   end
 
   def test_variant_any_implicit_render
     get :variant_any_implicit_render, params: { v: :tablet }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "tablet", @response.body
 
     get :variant_any_implicit_render, params: { v: :phablet }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phablet", @response.body
   end
 
   def test_variant_any_with_none
     get :variant_any_with_none
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "none or phone", @response.body
 
     get :variant_any_with_none, params: { v: :phone }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "none or phone", @response.body
   end
 
   def test_format_any_variant_any
     get :format_any_variant_any, format: :js, params: { v: :tablet }
-    assert_equal "text/javascript", @response.content_type
+    assert_equal "text/javascript", @response.media_type
     assert_equal "tablet", @response.body
   end
 
   def test_variant_negotiation_inline_syntax
     get :variant_inline_syntax_without_block, params: { v: [:tablet, :phone] }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
   end
 
   def test_variant_negotiation_block_syntax
     get :variant_plus_none_for_format, params: { v: [:tablet, :phone] }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
   end
 
   def test_variant_negotiation_without_block
     get :variant_inline_syntax_without_block, params: { v: [:tablet, :phone] }
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
     assert_equal "phone", @response.body
   end
 end
diff --git a/actionpack/test/controller/render_js_test.rb b/actionpack/test/controller/render_js_test.rb
index 1efc0b9de1..7ef35c2396 100644
--- a/actionpack/test/controller/render_js_test.rb
+++ b/actionpack/test/controller/render_js_test.rb
@@ -26,7 +26,7 @@ class RenderJSTest < ActionController::TestCase
   def test_render_vanilla_js
     get :render_vanilla_js_hello, xhr: true
     assert_equal "alert('hello')", @response.body
-    assert_equal "text/javascript", @response.content_type
+    assert_equal "text/javascript", @response.media_type
   end
 
   def test_should_render_js_partial
diff --git a/actionpack/test/controller/render_json_test.rb b/actionpack/test/controller/render_json_test.rb
index 82c1ba26cb..aba7593c15 100644
--- a/actionpack/test/controller/render_json_test.rb
+++ b/actionpack/test/controller/render_json_test.rb
@@ -80,7 +80,7 @@ class RenderJsonTest < ActionController::TestCase
   def test_render_json_nil
     get :render_json_nil
     assert_equal "null", @response.body
-    assert_equal "application/json", @response.content_type
+    assert_equal "application/json", @response.media_type
   end
 
   def test_render_json_render_to_string
@@ -91,7 +91,7 @@ class RenderJsonTest < ActionController::TestCase
   def test_render_json
     get :render_json_hello_world
     assert_equal '{"hello":"world"}', @response.body
-    assert_equal "application/json", @response.content_type
+    assert_equal "application/json", @response.media_type
   end
 
   def test_render_json_with_status
@@ -103,31 +103,31 @@ class RenderJsonTest < ActionController::TestCase
   def test_render_json_with_callback
     get :render_json_hello_world_with_callback, xhr: true
     assert_equal '/**/alert({"hello":"world"})', @response.body
-    assert_equal "text/javascript", @response.content_type
+    assert_equal "text/javascript", @response.media_type
   end
 
   def test_render_json_with_custom_content_type
     get :render_json_with_custom_content_type, xhr: true
     assert_equal '{"hello":"world"}', @response.body
-    assert_equal "text/javascript", @response.content_type
+    assert_equal "text/javascript", @response.media_type
   end
 
   def test_render_symbol_json
     get :render_symbol_json
     assert_equal '{"hello":"world"}', @response.body
-    assert_equal "application/json", @response.content_type
+    assert_equal "application/json", @response.media_type
   end
 
   def test_render_json_with_render_to_string
     get :render_json_with_render_to_string
     assert_equal '{"hello":"partial html"}', @response.body
-    assert_equal "application/json", @response.content_type
+    assert_equal "application/json", @response.media_type
   end
 
   def test_render_json_forwards_extra_options
     get :render_json_with_extra_options
     assert_equal '{"a":"b"}', @response.body
-    assert_equal "application/json", @response.content_type
+    assert_equal "application/json", @response.media_type
   end
 
   def test_render_json_calls_to_json_from_object
diff --git a/actionpack/test/controller/render_xml_test.rb b/actionpack/test/controller/render_xml_test.rb
index a72d14e4bb..7d61076e7c 100644
--- a/actionpack/test/controller/render_xml_test.rb
+++ b/actionpack/test/controller/render_xml_test.rb
@@ -92,11 +92,11 @@ class RenderXmlTest < ActionController::TestCase
 
   def test_should_render_xml_but_keep_custom_content_type
     get :render_xml_with_custom_content_type
-    assert_equal "application/atomsvc+xml", @response.content_type
+    assert_equal "application/atomsvc+xml", @response.media_type
   end
 
   def test_should_use_implicit_content_type
     get :implicit_content_type, format: "atom"
-    assert_equal Mime[:atom], @response.content_type
+    assert_equal Mime[:atom], @response.media_type
   end
 end
diff --git a/actionpack/test/controller/renderers_test.rb b/actionpack/test/controller/renderers_test.rb
index d92de6f5d5..96cce664a4 100644
--- a/actionpack/test/controller/renderers_test.rb
+++ b/actionpack/test/controller/renderers_test.rb
@@ -73,7 +73,7 @@ class RenderersTest < ActionController::TestCase
     assert_raise ActionView::MissingTemplate do
       get :respond_to_mime, format: "csv"
     end
-    assert_equal Mime[:csv], @response.content_type
+    assert_equal Mime[:csv], @response.media_type
     assert_equal "", @response.body
   end
 
@@ -83,7 +83,7 @@ class RenderersTest < ActionController::TestCase
     end
     @request.accept = "text/csv"
     get :respond_to_mime, format: "csv"
-    assert_equal Mime[:csv], @response.content_type
+    assert_equal Mime[:csv], @response.media_type
     assert_equal "c,s,v", @response.body
   ensure
     ActionController::Renderers.remove :csv
diff --git a/actionpack/test/controller/show_exceptions_test.rb b/actionpack/test/controller/show_exceptions_test.rb
index 1d68a359dc..2cd2114db6 100644
--- a/actionpack/test/controller/show_exceptions_test.rb
+++ b/actionpack/test/controller/show_exceptions_test.rb
@@ -76,7 +76,7 @@ module ShowExceptions
       @app = ShowExceptionsOverriddenController.action(:boom)
       get "/", headers: { "HTTP_ACCEPT" => "application/json" }
       assert_response :internal_server_error
-      assert_equal "application/json", response.content_type.to_s
+      assert_equal "application/json", response.media_type
       assert_equal({ status: 500, error: "Internal Server Error" }.to_json, response.body)
     end
 
@@ -84,7 +84,7 @@ module ShowExceptions
       @app = ShowExceptionsOverriddenController.action(:boom)
       get "/", headers: { "HTTP_ACCEPT" => "application/xml" }
       assert_response :internal_server_error
-      assert_equal "application/xml", response.content_type.to_s
+      assert_equal "application/xml", response.media_type
       assert_equal({ status: 500, error: "Internal Server Error" }.to_xml, response.body)
     end
 
@@ -92,7 +92,7 @@ module ShowExceptions
       @app = ShowExceptionsOverriddenController.action(:boom)
       get "/", headers: { "HTTP_ACCEPT" => "text/csv" }
       assert_response :internal_server_error
-      assert_equal "text/html", response.content_type.to_s
+      assert_equal "text/html", response.media_type
     end
   end
 
@@ -106,7 +106,7 @@ module ShowExceptions
 
       get "/", headers: { "HTTP_ACCEPT" => "text/json" }
       assert_response :internal_server_error
-      assert_equal "text/plain", response.content_type.to_s
+      assert_equal "text/plain", response.media_type
     ensure
       middleware.instance_variable_set(:@exceptions_app, @exceptions_app)
       $stderr = STDERR
diff --git a/actionpack/test/dispatch/debug_exceptions_test.rb b/actionpack/test/dispatch/debug_exceptions_test.rb
index 3e57e8f4d9..68817ccdea 100644
--- a/actionpack/test/dispatch/debug_exceptions_test.rb
+++ b/actionpack/test/dispatch/debug_exceptions_test.rb
@@ -208,7 +208,7 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
     assert_response 500
     assert_no_match(/<header>/, body)
     assert_no_match(/<body>/, body)
-    assert_equal "text/plain", response.content_type
+    assert_equal "text/plain", response.media_type
     assert_match(/RuntimeError\npuke/, body)
 
     Rails.stub :root, Pathname.new(".") do
@@ -222,31 +222,31 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
     get "/not_found", headers: xhr_request_env
     assert_response 404
     assert_no_match(/<body>/, body)
-    assert_equal "text/plain", response.content_type
+    assert_equal "text/plain", response.media_type
     assert_match(/#{AbstractController::ActionNotFound.name}/, body)
 
     get "/method_not_allowed", headers: xhr_request_env
     assert_response 405
     assert_no_match(/<body>/, body)
-    assert_equal "text/plain", response.content_type
+    assert_equal "text/plain", response.media_type
     assert_match(/ActionController::MethodNotAllowed/, body)
 
     get "/unknown_http_method", headers: xhr_request_env
     assert_response 405
     assert_no_match(/<body>/, body)
-    assert_equal "text/plain", response.content_type
+    assert_equal "text/plain", response.media_type
     assert_match(/ActionController::UnknownHttpMethod/, body)
 
     get "/bad_request", headers: xhr_request_env
     assert_response 400
     assert_no_match(/<body>/, body)
-    assert_equal "text/plain", response.content_type
+    assert_equal "text/plain", response.media_type
     assert_match(/ActionController::BadRequest/, body)
 
     get "/parameter_missing", headers: xhr_request_env
     assert_response 400
     assert_no_match(/<body>/, body)
-    assert_equal "text/plain", response.content_type
+    assert_equal "text/plain", response.media_type
     assert_match(/ActionController::ParameterMissing/, body)
   end
 
@@ -257,37 +257,37 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
     assert_response 500
     assert_no_match(/<header>/, body)
     assert_no_match(/<body>/, body)
-    assert_equal "application/json", response.content_type
+    assert_equal "application/json", response.media_type
     assert_match(/RuntimeError: puke/, body)
 
     get "/not_found", headers: { "action_dispatch.show_exceptions" => true }, as: :json
     assert_response 404
     assert_no_match(/<body>/, body)
-    assert_equal "application/json", response.content_type
+    assert_equal "application/json", response.media_type
     assert_match(/#{AbstractController::ActionNotFound.name}/, body)
 
     get "/method_not_allowed", headers: { "action_dispatch.show_exceptions" => true }, as: :json
     assert_response 405
     assert_no_match(/<body>/, body)
-    assert_equal "application/json", response.content_type
+    assert_equal "application/json", response.media_type
     assert_match(/ActionController::MethodNotAllowed/, body)
 
     get "/unknown_http_method", headers: { "action_dispatch.show_exceptions" => true }, as: :json
     assert_response 405
     assert_no_match(/<body>/, body)
-    assert_equal "application/json", response.content_type
+    assert_equal "application/json", response.media_type
     assert_match(/ActionController::UnknownHttpMethod/, body)
 
     get "/bad_request", headers: { "action_dispatch.show_exceptions" => true }, as: :json
     assert_response 400
     assert_no_match(/<body>/, body)
-    assert_equal "application/json", response.content_type
+    assert_equal "application/json", response.media_type
     assert_match(/ActionController::BadRequest/, body)
 
     get "/parameter_missing", headers: { "action_dispatch.show_exceptions" => true }, as: :json
     assert_response 400
     assert_no_match(/<body>/, body)
-    assert_equal "application/json", response.content_type
+    assert_equal "application/json", response.media_type
     assert_match(/ActionController::ParameterMissing/, body)
   end
 
@@ -298,7 +298,7 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
     assert_response 500
     assert_match(/<header>/, body)
     assert_match(/<body>/, body)
-    assert_equal "text/html", response.content_type
+    assert_equal "text/html", response.media_type
     assert_match(/puke/, body)
   end
 
@@ -307,7 +307,7 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
 
     get "/index.xml", headers: { "action_dispatch.show_exceptions" => true }
     assert_response 500
-    assert_equal "application/xml", response.content_type
+    assert_equal "application/xml", response.media_type
     assert_match(/RuntimeError: puke/, body)
   end
 
@@ -321,7 +321,7 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
 
     get "/index", headers: { "action_dispatch.show_exceptions" => true }, as: :wibble
     assert_response 500
-    assert_equal "application/json", response.content_type
+    assert_equal "application/json", response.media_type
     assert_match(/RuntimeError: puke/, body)
 
   ensure
diff --git a/actionpack/test/dispatch/response_test.rb b/actionpack/test/dispatch/response_test.rb
index 7758b0406a..33cf86a081 100644
--- a/actionpack/test/dispatch/response_test.rb
+++ b/actionpack/test/dispatch/response_test.rb
@@ -290,8 +290,8 @@ class ResponseTest < ActiveSupport::TestCase
     resp.to_a
 
     assert_equal("utf-16", resp.charset)
-    assert_equal(Mime[:xml], resp.content_type)
-
+    assert_equal(Mime[:xml], resp.media_type)
+    assert_equal("application/xml; charset=utf-16", resp.content_type)
     assert_equal("application/xml; charset=utf-16", resp.headers["Content-Type"])
   end
 
@@ -503,8 +503,8 @@ class ResponseIntegrationTest < ActionDispatch::IntegrationTest
     assert_response :success
 
     assert_equal("utf-16", @response.charset)
-    assert_equal(Mime[:xml], @response.content_type)
-
+    assert_equal(Mime[:xml], @response.media_type)
+    assert_equal("application/xml; charset=utf-16", @response.content_type)
     assert_equal("application/xml; charset=utf-16", @response.headers["Content-Type"])
   end
 
@@ -519,8 +519,8 @@ class ResponseIntegrationTest < ActionDispatch::IntegrationTest
     assert_response :success
 
     assert_equal("utf-16", @response.charset)
-    assert_equal(Mime[:xml], @response.content_type)
-
+    assert_equal(Mime[:xml], @response.media_type)
+    assert_equal("application/xml; charset=utf-16", @response.content_type)
     assert_equal("application/xml; charset=utf-16", @response.headers["Content-Type"])
   end
 
@@ -553,7 +553,26 @@ class ResponseIntegrationTest < ActionDispatch::IntegrationTest
     assert_response :success
 
     assert_equal("text/csv; charset=utf-16; header=present", @response.headers["Content-Type"])
-    assert_equal("text/csv", @response.content_type)
+    assert_equal("text/csv; charset=utf-16; header=present", @response.content_type)
+    assert_equal("text/csv", @response.media_type)
+    assert_equal("utf-16", @response.charset)
+  end
+
+  test "response Content-Type with optional parameters that set before charset" do
+    @app = lambda { |env|
+      [
+        200,
+        { "Content-Type" => "text/csv; header=present; charset=utf-16" },
+        ["Hello"]
+      ]
+    }
+
+    get "/"
+    assert_response :success
+
+    assert_equal("text/csv; header=present; charset=utf-16", @response.headers["Content-Type"])
+    assert_equal("text/csv; header=present; charset=utf-16", @response.content_type)
+    assert_equal("text/csv", @response.media_type)
     assert_equal("utf-16", @response.charset)
   end
 
@@ -570,7 +589,8 @@ class ResponseIntegrationTest < ActionDispatch::IntegrationTest
     assert_response :success
 
     assert_equal('text/csv; header=present; charset="utf-16"', @response.headers["Content-Type"])
-    assert_equal("text/csv", @response.content_type)
+    assert_equal('text/csv; header=present; charset="utf-16"', @response.content_type)
+    assert_equal("text/csv", @response.media_type)
     assert_equal("utf-16", @response.charset)
   end
 end
diff --git a/actionview/test/actionpack/controller/render_test.rb b/actionview/test/actionpack/controller/render_test.rb
index c8ce7366d1..9b1a720636 100644
--- a/actionview/test/actionpack/controller/render_test.rb
+++ b/actionview/test/actionpack/controller/render_test.rb
@@ -1003,14 +1003,14 @@ class RenderTest < ActionController::TestCase
   def test_render_xml
     get :render_xml_hello
     assert_equal "<html>\n  <p>Hello David</p>\n<p>This is grand!</p>\n</html>\n", @response.body
-    assert_equal "application/xml", @response.content_type
+    assert_equal "application/xml", @response.media_type
   end
 
   # :ported:
   def test_render_xml_as_string_template
     get :render_xml_hello_as_string_template
     assert_equal "<html>\n  <p>Hello David</p>\n<p>This is grand!</p>\n</html>\n", @response.body
-    assert_equal "application/xml", @response.content_type
+    assert_equal "application/xml", @response.media_type
   end
 
   # :ported:
@@ -1039,7 +1039,7 @@ class RenderTest < ActionController::TestCase
   def test_rendered_format_without_format
     get :inline_rendered_format_without_format
     assert_equal "test", @response.body
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_partials_list
@@ -1077,7 +1077,7 @@ class RenderTest < ActionController::TestCase
   def test_accessing_local_assigns_in_inline_template
     get :accessing_local_assigns_in_inline_template, params: { local_name: "Local David" }
     assert_equal "Goodbye, Local David", @response.body
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_should_implicitly_render_html_template_from_xhr_request
@@ -1264,13 +1264,13 @@ class RenderTest < ActionController::TestCase
   def test_partial_only
     get :partial_only
     assert_equal "only partial", @response.body
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_should_render_html_formatted_partial
     get :partial
     assert_equal "partial html", @response.body
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_render_html_formatted_partial_even_with_other_mime_time_in_accept
@@ -1279,20 +1279,20 @@ class RenderTest < ActionController::TestCase
     get :partial_html_erb
 
     assert_equal "partial.html.erb", @response.body.strip
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_should_render_html_partial_with_formats
     get :partial_formats_html
     assert_equal "partial html", @response.body
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_render_to_string_partial
     get :render_to_string_with_partial
     assert_equal "only partial", @controller.instance_variable_get(:@partial_only)
     assert_equal "Hello: david", @controller.instance_variable_get(:@partial_with_locals)
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_render_to_string_with_template_and_html_partial
@@ -1300,21 +1300,21 @@ class RenderTest < ActionController::TestCase
     assert_equal "**only partial**\n", @controller.instance_variable_get(:@text)
     assert_equal "<strong>only partial</strong>\n", @controller.instance_variable_get(:@html)
     assert_equal "<strong>only html partial</strong>\n", @response.body
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_render_to_string_and_render_with_different_formats
     get :render_to_string_and_render_with_different_formats
     assert_equal "<strong>only partial</strong>\n", @controller.instance_variable_get(:@html)
     assert_equal "**only partial**\n", @response.body
-    assert_equal "text/plain", @response.content_type
+    assert_equal "text/plain", @response.media_type
   end
 
   def test_render_template_within_a_template_with_other_format
     get :render_template_within_a_template_with_other_format
     expected = "only html partial<p>This is grand!</p>"
     assert_equal expected, @response.body.strip
-    assert_equal "text/html", @response.content_type
+    assert_equal "text/html", @response.media_type
   end
 
   def test_partial_with_counter
diff --git a/guides/source/6_0_release_notes.md b/guides/source/6_0_release_notes.md
index fa45e7240d..e421ae1ac7 100644
--- a/guides/source/6_0_release_notes.md
+++ b/guides/source/6_0_release_notes.md
@@ -215,6 +215,10 @@ Please refer to the [Changelog][action-pack] for detailed changes.
 
 ### Notable changes
 
+*   Change `ActionDispatch::Response#content_type` returning Content-Type
+    header as it is.
+    ([Pull Request](https://github.com/rails/rails/pull/36034))
+
 *   Raise an `ArgumentError` if a resource param contains a colon.
     ([Pull Request](https://github.com/rails/rails/pull/35236))
 
diff --git a/guides/source/testing.md b/guides/source/testing.md
index 9540bb2af5..41bc54b924 100644
--- a/guides/source/testing.md
+++ b/guides/source/testing.md
@@ -1144,7 +1144,7 @@ test "ajax request" do
   get article_url(article), xhr: true
 
   assert_equal 'hello world', @response.body
-  assert_equal "text/javascript", @response.content_type
+  assert_equal "text/javascript", @response.media_type
 end
 ```
 
diff --git a/guides/source/upgrading_ruby_on_rails.md b/guides/source/upgrading_ruby_on_rails.md
index b8a5c39f39..1110592d5e 100644
--- a/guides/source/upgrading_ruby_on_rails.md
+++ b/guides/source/upgrading_ruby_on_rails.md
@@ -134,6 +134,28 @@ Action Cable JavaScript API:
   +    ActionCable.logger.enabled = false
   ```
 
+### `ActionDispatch::Response#content_type` now returned Content-Type header as it is.
+
+Previously, `ActionDispatch::Response#content_type` returned value does NOT contain charset part.
+This behavior changed to returned Content-Type header containing charset part as it is.
+
+If you want just MIME type, please use `ActionDispatch::Response#media_type` instead.
+
+Before:
+
+```ruby
+resp = ActionDispatch::Response.new(200, "Content-Type" => "text/csv; header=present; charset=utf-16")
+resp.content_type #=> "text/csv; header=present"
+```
+
+After:
+
+```ruby
+resp = ActionDispatch::Response.new(200, "Content-Type" => "text/csv; header=present; charset=utf-16")
+resp.content_type #=> "text/csv; header=present; charset=utf-16"
+resp.media_type   #=> "text/csv"
+```
+
 ### Autoloading
 
 The default configuration for Rails 6