Bug report:
I did not see a place to file a GitHub issue for fedibird so I write it here.
The WebP support is incomplete. There should be a thumbnail image generated, but there isn't.
So after uploading a WebP image, the compose form looks like the second pic:

(The first pic is the WebP image uploaded to fedibird.)

Fedibird

様々な目的に使える、日本の汎用マストドンサーバーです。安定した利用環境と、多数の独自機能を提供しています。