Issues with tickets and mp4 files

When i open a ticket / posting here and upload a *.mp4 (845 KB) file it IS accepted and appears as attachment. When I try to press save the site reloads but the attachment is magically removed and I need to press again on save to update the ticket / posting (but then without the *.mp4 attachment). I also do not see an error message or something which say that *.mp4 files aren´t allowed. Can you check that?

I´m using the latest firefox browser. I will also add the affected *.mp4 so you can use that to reproduce the issue on your side.