Upload a File broken

Every time I click "Upload a File" and pick a file, a window pops up saying:

The following error occurred

There was a problem uploading your file.

I've tried uploading from Chrome on Windows, Firefox on Ubuntu, and Chrome and Safari on OSX. When I click Upload a File on Safari (and one time in Chrome), instead of being immediately asked to pick a file to upload, I get an extra window like this:

Upload a file from your computer (Max 5 MB):
[Choose File] no file selected [Close]
Accepted file types: zip, txt, pdf, png, jpg, jpeg, jpe, gif, xlsx
Max image width: 960 pixels
Max image height: 720 pixels

If I choose a file, sometimes it briefly appears in place of "no file selected" and then goes back to saying "no file selected". If I try again, the file name remains there. The only thing I can do at that point is click [Close] and then nothing happens - no attached file appears with the post.

I've resized the picture I'm trying to upload and tried jpg and png. The smallest version I've tried is 37k 453x360.

This problem was happening last night and today.