@Eegore8a MKV support was added at platform level, so will work for phones and PCs (same code!) This is a big advantage of common platform.
— Gabriel Aul (@GabeAul) February 12, 2015
We had included native FLAC and MKV support in our Windows 10 expected features list, before Windows 10 for Phones was officially unveiled. Reason is quite simple, Windows 10 Desktop already had it and the underlying OS core being the same, we thought it will be present in Windows 10 for Phones too.
Thanks Nate for the tip. Cheers!!