[chore] Thumbnail only first frame of animated media

[tsmethurst]

Description

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Updates our ffmpeg call to just take the first frame and not use the thumbnail filter, which was giving unexpected results for thumbnails of short media.

We could probably do some more nuanced logic here (check the first 1% of frames if media is longer than x amount of time) but I don't think it's that necessary right now.

Closes #3447

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[Ryuno-Ki]

I'm curious: If the animated started black, would it pick those?

There's a filter in regular ffmpeg: https://scribe.rip/@georgechmr/skipping-black-frame-when-generating-video-thumbnail-with-ffmpeg-9e26bf2f67e3

[tsmethurst]

We could use something like that black filter in future maybe, but I don't think it's urgent right now. We also have to consider processing + memory costs when doing that kind of analysis, so something to benchmark a little bit (even anecdotally).