[BlueSky] Webp files disallowed, likely as an oversight.

[TheBeastintheTable]

Bluesky readily accepts webp files from the web interface. This could either be a problem on their end (the API not supporting the same formats the web interface does) or a problem on the postybirb side (mistakenly not having that under the allowed formats list for bsky)

If the problem is the latter, this should be an easy fix. If the former, I suppose I gotta start converting things.