MKV support

MKV is a video container that can support both the codecs of mp4 and webm together. The MKV container is exactly the same as the webm container, it's just that webm only supports open source codecs. MKV kind of already works, if you rename the file extention to .webm it works even if it uses other codecs not supported by webm (as long as those codecs are supported by discord e.g. h.264, aac).

It probably wouldn't be hard to add "official" support for it.

It would also be nice if discord supported MKA which MKVs audio format. Though those also work by renaming it to .ogg.