3 files changed, 17 insertions, 4 deletions
diff --git a/actionpack/lib/action_dispatch/testing/integration.rb b/actionpack/lib/action_dispatch/testing/integration.rb
index 742bce1ca6..8a8e22053a 100644
--- a/actionpack/lib/action_dispatch/testing/integration.rb
+++ b/actionpack/lib/action_dispatch/testing/integration.rb
@@ -381,7 +381,7 @@ module ActionDispatch
           response = _mock_session.last_response
           @response = ActionDispatch::TestResponse.from_response(response)
           @response.request = @request
-          @response.response_parser = request_encoder
+          @response.response_parser = RequestEncoder.parser(@response.content_type)
           @html_document = nil
           @url_options = nil
 
@@ -397,6 +397,8 @@ module ActionDispatch
         class RequestEncoder # :nodoc:
           @encoders = {}
 
+          attr_reader :response_parser
+
           def initialize(mime_name, param_encoder, response_parser, url_encoded_form = false)
             @mime = Mime[mime_name]
 
@@ -424,8 +426,9 @@ module ActionDispatch
             @param_encoder.call(params)
           end
 
-          def parse_body(body)
-            @response_parser.call(body)
+          def self.parser(content_type)
+            mime = Mime::Type.lookup(content_type)
+            encoder(mime ? mime.ref : nil).response_parser
           end
 
           def self.encoder(name)
diff --git a/actionpack/lib/action_dispatch/testing/test_response.rb b/actionpack/lib/action_dispatch/testing/test_response.rb
index 58d3e6eb0f..4f289ad4b5 100644
--- a/actionpack/lib/action_dispatch/testing/test_response.rb
+++ b/actionpack/lib/action_dispatch/testing/test_response.rb
@@ -22,7 +22,7 @@ module ActionDispatch
     attr_writer :response_parser # :nodoc:
 
     def parsed_body
-      @response_parser.parse_body(body)
+      @response_parser.call(body)
     end
   end
 end
diff --git a/actionpack/test/controller/integration_test.rb b/actionpack/test/controller/integration_test.rb
index cb524bacb2..ea50f05f4d 100644
--- a/actionpack/test/controller/integration_test.rb
+++ b/actionpack/test/controller/integration_test.rb
@@ -1171,6 +1171,16 @@ class IntegrationRequestEncodersTest < ActionDispatch::IntegrationTest
     Mime::Type.unregister :wibble
   end
 
+  def test_parsed_body_without_as_option
+    with_routing do |routes|
+      routes.draw { get ':action' => FooController }
+
+      get '/foos_json.json', params: { foo: 'heyo' }
+
+      assert_equal({ 'foo' => 'heyo' }, response.parsed_body)
+    end
+  end
+
   private
     def post_to_foos(as:)
       with_routing do |routes|