When I receive a message with a video file attached Thunderbird 91.1.1 will ask me for the application to open it with. I select the application I want to use and tick the box always use that application. If I get another message(s) with the same type of attachment Thunderbird will ask me again to choose the application even though I said to always use the same one I picked earlier. If I check in general preferences of Thunderbird it shows the application I have chosen as the default for the file type.

I'm running Win 10 64Bit 21H1 VLC 3.0.16 Thunderbird 91.1.1 (32bit)