Tom -- Some files have in the past been reported to cause a "broken-link" type thing to appear in the dialog box, trying to render a thumbnail image. But that was a cosmetic problem; if you subsequently clicked on the "Add" button the upload proceeded anyway.
But for that FITS file, I tried it too, and had to reboot my computer to get a browser window (Chrome) back.
So this is a new one on me, and I suspect only support can help you. Be sure to tag it with a #bug hashtag to catch his eye. Some things can linger in the queue for quite some time after initial triage, but he's usually pretty quick about fixing actual bugs.
Regards,
Bruce
Bruce Bowman