'success' event missing second argument

'success' event should return a server response as a second argument. Now it returns only a file.

`success?: FileCallback | FileCallback[];`

http://www.dropzonejs.com/#event-success

Locate the success callback type and the event implementation in the JavaScript project, then compare them with Dropzone.js's documented success event. Confirm that the callback receives both the uploaded file and the server response, and update the relevant behavior and coverage so the second argument is present.

Written by the indexing model from the issue text.