Attaching files to a bug report while filing it never stops the "Uploading" progress bar

Not reproducible now in this bug, but it happened in bug 2849. Reproduction steps show what I did there.

Reproduction:
1. Create a new bug ("Submit" link on the right bar)
2. Fill out fields
3. Click "Browse..." button next to "Attach new file:"
4. Select plaintext file (foo.txt) with 53 KB size, OK
5. Click "Attach" button

Actual result:
1. Attachment part turns into a pseudo progress bar (in HTML) saying "Uploading".
2. It never stops (waited a few minutes)
3. If I click "Preview" regardless:
4. Preview page shows two files (both with foo.txt as filename) attached, only the second one has "Listed" checkbox checked.
5. I check "Delete" checkbox for the first file, leaving the second as-is (not Deleted, but "Listed")
6. I click "Preview" again, and get the same page again.
7. I click "Submit"
8. The file foo.txt appears in the bug report, and can be opened.

Expected result:
1. "Uploading" bar disappears after a few seconds, file is shown only once, with "Listed" checkbox checked.