Allow uploading mkv files

[baryluk]

Just like mov or mp4 are already supported, mkv should be supported. mkv is a default format produced by obs for example, and it is often useful to create few second clips to show issues when reporting bugs.

If it can't be played by embedded player, it still should be possible to download such files.

[iquat]

I've just ran into this issue with OBS recording something irreversible happening on my screen (see notepad-plus-plus/notepad-plus-plus#12473)

[iquat]

Workaround within OBS: record in .mkv, and then remux into .mp4 (File -> Remux Recordings)

[AlexFolland]

Compared to MP4, MKV has more features, compatibility, and better tools (especially with the existence of the mkvtoolnix GUI). One of the best features of MKV over MP4 is that the video can be played before encoding is complete, enabling getting a quick preview before committing to the full encoding time. Currently I'm using MKV during encoding and then remuxing to MP4 after the encoding is complete just for attaching to github, but I would prefer to avoid the final step since MKV is the more modern and useful format overall. Fixing this seems like it would be a relatively simple change; basically just adding "mkv" to the file extension allow list. Thanks.