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"Crappy Design" as in I can't view the video?
No video with supported format and MIME type found.
I think that's the fault of whatever app / frontend you are using. On OP's instance (which is just rendering HTML), and on the community's instance (using lemmy-ui), this file plays. In both cases, it's a bog-standard MP4 inside a VIDEO tag with a video/mp4 type. If - for example - you're using Sync, then that app also can't play MP4s hosted by Mastodon either.
In voyager the video plays if you click on the thread and view the video there. It doesn’t show the video if you just click from the main page. That gave me the error op had.
Interesting, thanks. I can see the video if I view via voyager here: https://m.lemmy.world/search/lemmy.world/c/crappydesign@sh.itjust.works
That it displays in some views but not others though, reinforces (to me) that the problem is with the app, not the post. I'd gently suggest (to the person I originally replied to) that if something isn't displaying, the first thing to do is see what it looks like in a browser, and if that's okay, report the problem to the app author, not reply to the OP. We should all just be able to post stuff, without worrying about every feasible way that someone might want to view it.
Checked with your instance (feddit.nl), community's instance (sh.itjust.works), and with lemmy.world. None show a thumbnail, but all can play the video (at least on default interface).
Indeed, for some reason, the video cannot be viewed when opened in another browser tab