MP4 gifs on discord - How does it work?

I've noticed gifs on discord load really fast and are super high quality. No wonder! They're actually MP4s. Videos. Even linking directly to a .gif on imgur, makes discord show the MP4 version.

This is pretty great - saves a TON of data. But HOW is it done, from the websites side? I need to do this too.

PS. It'd be great if you could do this when just sending a file and if it worked in bot embeds as well.