Currently only jpg, jpeg, png, gif, zip attachments are allowed on posts which lead to me converting my 80kB WebM to a 130kB and worse looking gif 
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Pretty sure they're using https://www.discourse.org/
Not sure they have a lot of control over it.
I believe the Discourse official line is that support is still patchy: https://caniuse.com/webm
We can allow new filetypes, but we’ve been hesitant to allow video, since the files are usually ordered of magnitude larger than anything else. Think of our poor server 
Of course, in your example, the WebM would be very small, so perhaps there’s some combination of filetype and file size we could look at…
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I think Discourse does a pretty good job displaying things in-line when hosted else where:
Test of random webm video:
This won't play on my iPhone 
