You can see the full list of changes I've made here:https://github.com/ccd0/Tinyboard/compare/savetheinternet:master...master#files_bucket
This includes what I've done to the templates.
The problem I've seen with just simply using the <video> tag with preload="auto" is that browsers tend to download the entire video if it's relatively short. What's being done here is to extract one frame from the video and use that as the "thumbnail," which isn't ideal but saves some bandwidth over the simplest way of doing things.
If you can run FFmpeg, you should use that to create actual thumbnails, which is better than what's done here. It isn't used here because I wanted to have a baseline that anyone can install, and not everyone hosting a site manages their own VPS. I'm going to add it as an option in containerchan once I have a bit more time to spend on it.