5 files changed, 27 insertions, 2 deletions
diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index cfefeb23ce..8181b386be 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,12 @@
+*   Fix header `Content-Type: #<Mime::NullType:...>` in localized template.
+
+    When localized template has no format in the template name,
+    the response now has the default and correct `content-type`.
+
+    Fixes #13064.
+
+    *Angelo Capilleri*
+
 *   Try to escape each part of a url correctly when using a redirect route.
 
     Fixes #13110.
diff --git a/actionpack/lib/action_controller/metal/rendering.rb b/actionpack/lib/action_controller/metal/rendering.rb
index 5c48b4ab98..66d34f3b67 100644
--- a/actionpack/lib/action_controller/metal/rendering.rb
+++ b/actionpack/lib/action_controller/metal/rendering.rb
@@ -34,7 +34,8 @@ module ActionController
 
     def _process_format(format)
       super
-      self.content_type ||= format.to_s
+      # format is a Mime::NullType instance here then this condition can't be changed to `if format`
+      self.content_type ||= format.to_s unless format.nil?
     end
 
     # Normalize arguments by catching blocks and setting them on :update.
diff --git a/actionpack/lib/action_dispatch/http/mime_type.rb b/actionpack/lib/action_dispatch/http/mime_type.rb
index c92d997a5c..a398919ca7 100644
--- a/actionpack/lib/action_dispatch/http/mime_type.rb
+++ b/actionpack/lib/action_dispatch/http/mime_type.rb
@@ -1,4 +1,5 @@
 require 'set'
+require 'singleton'
 require 'active_support/core_ext/module/attribute_accessors'
 require 'active_support/core_ext/string/starts_ends_with'
 
@@ -27,7 +28,7 @@ module Mime
   class << self
     def [](type)
       return type if type.is_a?(Type)
-      Type.lookup_by_extension(type) || NullType.new
+      Type.lookup_by_extension(type) || NullType.instance
     end
 
     def fetch(type)
@@ -292,6 +293,8 @@ module Mime
   end
 
   class NullType
+    include Singleton
+
     def nil?
       true
     end
diff --git a/actionpack/test/controller/localized_templates_test.rb b/actionpack/test/controller/localized_templates_test.rb
index 6b02eedaed..c95ef8a0c7 100644
--- a/actionpack/test/controller/localized_templates_test.rb
+++ b/actionpack/test/controller/localized_templates_test.rb
@@ -34,4 +34,15 @@ class LocalizedTemplatesTest < ActionController::TestCase
     get :hello_world
     assert_equal "Gutten Tag", @response.body
   end
+
+  def test_localized_template_has_correct_header_with_no_format_in_template_name
+    old_locale = I18n.locale
+    I18n.locale = :it
+
+    get :hello_world
+    assert_equal "Ciao Mondo", @response.body
+    assert_equal "text/html",  @response.content_type
+  ensure
+    I18n.locale = old_locale
+  end
 end
diff --git a/actionpack/test/fixtures/localized/hello_world.it.erb b/actionpack/test/fixtures/localized/hello_world.it.erb
new file mode 100644
index 0000000000..9191fdc187
--- /dev/null
+++ b/actionpack/test/fixtures/localized/hello_world.it.erb
@@ -0,0 +1 @@
+Ciao Mondo
+\ No newline at end of file