I would first try to get the source code in a similar way to the Limited File Uploads section. Once you have the upload.php source code you should be able to see what is happening on the back end. The image file gets modified during the upload process.
Take a look at the source code of upload.php and you will see.
-onthesauce
Can someone tell me which wordlist was used to get the user_feedback_submissions directory ? there is no hint in the website about its location. I fuzzed with raft-directory-large wordlist of seclists + the directories-big-2.3 wordlist but nothing …