Reported by Gryllida on commons:
Steps to reproduce:
*Go to https://commons.m.wikimedia.org/wiki/Special:Uploads
*Select contribute an image
*In the file selector, select a file that is in a format not allowed at commons, or a file that would otherwise be rejected (e.g. A JPG file that mime type detects to something that's not a jpg)
*User is presented with an error. Something along the lines of "Invalid file, please try something else. Valid formats are X"
*User is immediately returned to the list of images they had previously uploaded, with no indication what went wrong, or even if anything went wrong.