Uploading with Firefox fails

Some uploads using the latest Firefox version are corrupted. The upload seems to succeed, but downloading the file fails.

Details

OS: Ubuntu 20.04.1 LTS x86_64
Firefox: 83.0a1 (2020-10-14) (Nightly)
Host: send.vis.ee

Reproduce

• Upload this file in Firefox: https://archive.org/download/BigBuckBunny/big_buck_bunny_720p_h264.mov
• Try to download the file, download fails.

Additional notes

• Works fine in stable Firefox (81.0.2 (64-bit) on the same system)
• Works fine in other browsers such as Chrome
• This issue happens on a hosted instance, using send-docker-compose (send.vis.ee)
• Cannot reproduce this with send-docker-compose running locally (HTTP/80)
• Cannot reproduce this locally with a dev server (npm start)
• Firefox bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1672185

Related issue: ffsend#105 (closed)