Always render attachment partials as HTML with :html format inside trix editor

actiontext/lib/action_text/attachments/trix_conversion.rb

@@ -28,7 +28,7 @@ module ActionText
       private
         def trix_attachment_content
           if partial_path = attachable.try(:to_trix_content_attachment_partial_path)
-            ActionText::Content.render(partial: partial_path, object: self, as: model_name.element)
+            ActionText::Content.render(partial: partial_path, formats: :html, object: self, as: model_name.element)
           end
         end
     end

actiontext/test/dummy/app/views/messages/edit.json.erb

@@ -0,0 +1,4 @@
+{
+  "id": <%= @message.id %>,
+  "form": "<%= j render partial: "form", formats: :html, locals: { message: @message } %>"
+}

actiontext/test/integration/controller_render_test.rb

@@ -24,6 +24,15 @@ class ActionText::ControllerRenderTest < ActionDispatch::IntegrationTest
     assert_select content, "img:match('src', ?)", %r"//loocalhoost/.+/racecar"
   end
 
+  test "renders Trix with content attachment as HTML when the request format is not HTML" do
+    message_with_person_attachment = messages(:hello_alice)
+
+    get edit_message_path(message_with_person_attachment, format: :json)
+
+    form_html = response.parsed_body["form"]
+    assert_match %r" class=\S+mentionable-person\b", form_html
+  end
+
   test "resolves partials when controller is namespaced" do
     blob = create_file_blob(filename: "racecar.jpg", content_type: "image/jpg")
     message = Message.create!(content: ActionText::Content.new.append_attachables(blob))