I have further information on the previous bug: If you add a version to a mod, and you enter a description, the file uploads but the version is not displayed in the list of versions. If you leave the description empty, everything works.
New bug: When I upload a file as a version of a mod, sometimes the file gets renamed with a "1" appended to the end, even though the original did not have this. I speculate it's to prevent name collisions on the back-end; This is a problem because usually the end of my filenames are the version numbers, and this makes the version number wrong.