2 files changed, 30 insertions, 2 deletions
diff --git a/actionpack/lib/action_controller/test_case.rb b/actionpack/lib/action_controller/test_case.rb
index 472bb74add..fbbaa1a887 100644
--- a/actionpack/lib/action_controller/test_case.rb
+++ b/actionpack/lib/action_controller/test_case.rb
@@ -78,7 +78,9 @@ module ActionController
           # params parser middleware, and we should remove this roundtripping
           # when we switch to caling `call` on the controller
 
-          case content_mime_type.ref
+          case content_mime_type.to_sym
+          when nil
+            raise "Unknown Content-Type: #{content_type}"
           when :json
             data = ActiveSupport::JSON.encode(non_path_parameters)
             params = ActiveSupport::JSON.decode(data).with_indifferent_access
@@ -90,7 +92,8 @@ module ActionController
           when :url_encoded_form
             data = non_path_parameters.to_query
           else
-            raise "Unknown Content-Type: #{content_type}"
+            data = non_path_parameters.to_query
+            self.request_parameters = non_path_parameters
           end
         end
 
diff --git a/actionpack/test/controller/test_case_test.rb b/actionpack/test/controller/test_case_test.rb
index b3c3979c84..06bf9dec74 100644
--- a/actionpack/test/controller/test_case_test.rb
+++ b/actionpack/test/controller/test_case_test.rb
@@ -627,6 +627,31 @@ XML
     assert_equal "application/json", parsed_env["CONTENT_TYPE"]
   end
 
+  def test_mutating_content_type_headers_for_plain_text_files_sets_the_header
+    @request.headers['Content-Type'] = 'text/plain'
+    post :render_body, params: { name: 'foo.txt' }
+
+    assert_equal 'text/plain', @request.headers['Content-type']
+    assert_equal 'foo.txt', @request.request_parameters[:name]
+    assert_equal 'render_body', @request.path_parameters[:action]
+  end
+
+  def test_mutating_content_type_headers_for_html_files_sets_the_header
+    @request.headers['Content-Type'] = 'text/html'
+    post :render_body, params: { name: 'foo.html' }
+
+    assert_equal 'text/html', @request.headers['Content-type']
+    assert_equal 'foo.html', @request.request_parameters[:name]
+    assert_equal 'render_body', @request.path_parameters[:action]
+  end
+
+  def test_mutating_content_type_headers_for_non_registered_mime_type_raises_an_error
+    assert_raises(RuntimeError) do
+      @request.headers['Content-Type'] = 'type/fake'
+      post :render_body, params: { name: 'foo.fake' }
+    end
+  end
+
   def test_id_converted_to_string
     get :test_params, params: {
       id: 20, foo: Object.new