Feature Request: Add support for mkv videos

**Is your feature request related to a problem? Please describe.**
I frequently have mkv video material (e.g. from youtube-dl) that I'd like to share via Matrix but Element Android (and Element Web) doesn't handle recognize mkvs as videos and instead treats them as "files".

**Describe the solution you'd like**
mkvs are treated as first-class video citizens, just like mp4s.

**Describe alternatives you've considered**
The videos could be converted before sending them. Currently this must be done by hand but a converter could also be built in to Element Android.