Disable format determination based on path extension

config/initializers/action_dispatch_http_mime_negotiation.rb

+# Starting with Rails 5, Rails tries to determine the request format based on
+# the extension of the full URL path if no explicit `format` param or `Accept`
+# header is provided, like when simply browsing to a page in your browser.
+#
+# This is undesireable in GitLab, because many of our paths will end in a ref or
+# blob name that can end with any extension, while these pages should still be
+# presented as HTML unless otherwise specified.
+
+# We override `format_from_path_extension` to disable this behavior.
+
+module ActionDispatch
+  module Http
+    module MimeNegotiation
+      def format_from_path_extension
+        nil
+      end
+    end
+  end
+end

spec/controllers/projects/blob_controller_spec.rb

@@ -35,6 +35,11 @@ describe Projects::BlobController do
         let(:id) { 'binary-encoding/encoding/binary-1.bin' }
         it { is_expected.to respond_with(:success) }
       end
+
+      context "Markdown file" do
+        let(:id) { 'master/README.md' }
+        it { is_expected.to respond_with(:success) }
+      end
     end
 
     context 'with file path and JSON format' do