@Paladox: Have you tried yourself? Do you have specific upstream revisions in mind (that you could identify by URLs) that might have touched this behavior, which make it reasonable to assume that the problem might have improved?
Did Krenair explicitly agree to be added as a subscriber in T127564#2588490 ?
Cannot reproduce (anymore): Tried the same for a 6.4MB image on this instance. Got an error message (No configured storage engine can store this file. See "Configuring File Storage" in the documentation for information on configuring storage engines.).
(That error itself is covered in T155130: Unclear error message when uploading a larger attachment: "Exception: No configured storage engine can store this file.".)