Whoops, sorry about that. I meant to add that I was trying to file expenses on 84.0a1 (2020-11-02) (64-bit) and the files wouldn't upload either when I dragged and dropped them, or when I opened the file selector and chose one of the files. The website spun for a while, then popped up an error message talking about a possible invalid character or encoding, and didn't show the file. Turning Fission off lets me upload the files just fine.

Jens, this is pretty severe. Could you assign this to someone in the DOM core team to fix this asap?

Hmm, I used to reproduce this on the nightly build around 2020-11-03, but now I could not repdouce on the today's Nightly, 20201105045247. Blake, do you still see this bug on today's Nightly. If you still see the issue, can you post a video? Thanks!

Edit: I rechecked again, actually I could not reproduce this on the nightly build around 2020-11-03, either.

I can't seem to reproduce it on today's nightly either. So, uh, maybe it was fixed?

Not sure. I tried to find a possible fix or regression via mozregression, but failed.
Since we didn't seem to reproduce it, let's close as WORKSFORME first.
We could reopen or file a new one if we see this issue again. Thanks!