api: Properly handle case when thumbnail can't be generated
For some images - gifs in particular - thumbnails can't be
generated by Fractal. In such cases
attach_message(), however, we unwrapped
extra_content assuming it would never be
caused a crash when uploading gifs.
This commit makes sure that we handle the case of missing
extra_content by making
Option<ExtraContent>, then setting the thumbnail url
Incidentally, this commit also seems to fix an issue where images would show broken thumbnails after a user uploaded them.