I'm trying to help my client upload a video file as an attachment to a card so that they're able to play the video in the card without having to copy/paste the URL on their web browser. They tell me that they're able to play Youtube clips in a card, but I can't imagine that's the only accepted native video player that works in Trello.
This is what I've been able to do so far, but obviously not seeing any kind of preview/player here:
