Seeing as the file can't be deleted (at least not right away) is there a place one can test how will the video look and behave after uploading/webm convertion?

Post 3037952 was originally an .avi file with good quality and no freezing between loops and it was 503KB in size. I renamed it to .webm locally and it still plays fine, but what would happen if I upload it after the renaming, will it still convert to "proper" .webm and will it play without freezing?

Turns out converting it to anything other than .avi makes it freeze between loops, so renaming is the only way to prevent that.

Video uploads aren't renamed to .webm. They're converted, since WebM uses a completely different codec. It doesn't do this based on the extension, but rather the actual type it finds after analyzing the video.

The only thing you can do is convert the video to WebM yourself and upload that if it works out. We do not convert videos that are already WebM's. Like I said, though, it has to actually be a WebM.

Just use some good converter for making webm. For example, Freemake video converter, it is very simple and easy to understand

Freemake is kinda limited for webm. Doesn't allow me to control framerate and bitrate among other things, while it allows control over mp4. Finally managed to convert the video to webm without freezing and now it keeps saying "bad hash" when I try to upload it.

Upload as AVI with a high bitrate.

Did so. Good news is it doesn't freeze between loops, bad news is the quality is the same as the last time I tried uploading it.
gelbooru.com/index.php?page=post&s=view&id=3046607
29.1MB got converted to 175KB.
Is it possible to upload the .webm that's only 394KB and looks and loops exactly as expected somewhere else and for someone to look at it and say what's wrong with it?