Video attachment playback issues

[olivercoad]

When trying to play video attachments in team chat, it's very slow to load, if it loads at all. It also doesn't buffer properly. It plays half a second before spinning for a few seconds to load the next half second at a time.

Also as a side-note, it would be nice to be able to play videos in fullscreen.

Tested on desktop and android clients.

[dahu33]

Same problem here. Both tested from France and Japan.