Right now Rocket.Chat support playback of user uploaded webm videos in conversations, this works both through a browser and the desktop application (since it essentially is a borderless browser).
But it does not work in iOS. The uploaded webm file is noted as a video, and you can click on it and click play, but it gets stuck at the loading spinner, nothing happens.
This is obviously because of lack of support, not a bug. It would be great to have added support, in the very least, support of webm videos encoded in formats that do not require paid licensing.