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My understanding is that imgur wanted to accept videos. But since its audience is mentally handicapped (i guess?) they can't handle the idea of having mp4's without audio be the allowed video format. So they made a new extension which rendering/conversion vendors can target. Instead of having users convert their files to mp4 and leaving the audio channel, trying to upload and getting frustrated.
But perhaps they are just stripping the audio, that'd be the saner choice. Maybe the extension is just there to make it clear it's a file without audio.
It's a web compatible format made for marketing in some form anyhow. It doesn't do anything new.
