Seems like video files don’t upload properly to the forum.
More specifically, they appear to start uploading, the upload progresses properly, but it gets stuck at 100% and never completes.
This may or may not affect all video files, but at least mp4 files are affected. I tried this on Chrome (Version 136.0.7103.114) and LibreWolf (Version 136.0-2) (modified firefox).
I tried two different but similar files. Both small mp4 <10MB <1min screen recordings. Both get stuck at 100%.
Images upload just fine.