It might have been an issue with my set-up. I was trying to use the Jellyfin web version from my computer, which probably was not serving the files correctly. I’ll have to test again soon to see whether fixes were made in the last 2 years for it :)

Never mind. It’s because I was using Firefox, which doesn’t support x265 due to licensing issues.

Wait, does the pi4 support 265 now? I meddled around with it a few years ago, and it always had to transcode 265 to 264.

To offload the storage requirements of the instance you’re posting in. If everyone starts uploading images directly in the comment, the server HDD/SDD will be overloaded. Consider using a third party service and posting the link from there instead!