That's weird, why is this 600x338 and a gif file?

When danbooru fetches the media of https://twitter.com/i/web/status/1307430292240572417
(you can see this by attempting to batch upload the tweet)
it gets https://video.twimg.com/tweet_video/EiTsykvVkAE_C_E.mp4
which is 1280x720 and an mp4

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