Right now, the player only looks for mp4, webm, or m3u8 extensions. I'll test if m4v and mov are streamable containers. I don't think mkv is a streamable container.
Doesn't it depend on the type of codec the video file inside the MKV is using? I am able to stream MKV from a download link in DeoVR on my Quest 2 with random access, of course audio is still a problem if it is in AC3 or DTS.
Here are some internet archive video files that I can stream from download links on DeoVR but not HereSphere:
m4v: https://archive.org/download/homealone-opening-closing-vhs/02%20-%20Home%20Alone..
mov: https://archive.org/download/NASA_Launchpad_Biofeedback_HD/NASA_Launchpad_Biofee...
mkv: https://archive.org/download/411vm-issue-10-1995-vhs-deinterlaced-mkv-file/411VM...
mirors: http://164.90.192.193:84/test/02%20-%20Home%20Alone%20opening.m4v
http://164.90.192.193:84/test/NASA_Launchpad_Biofeedback_HD.mov
http://164.90.192.193:84/test/411VM%20-%20Issue%2010%20(1995)%20(VHS)%20%5b540p6...