I am seeing so many entries on this from earlier versions, and I am also seeing a lot of bug trackers on the issue. Strangely, I was able to upload jpeg's, and now it is telling me I can't. I get the exact message that Joseph got (first entry) in this link: https://moodle.org/mod/forum/discuss.php?d=203528
This wasn't there before - I had uploaded several files. Something changed, and I can't figure out what that is.