OK this is funny. If you have a folder in your pasteboard and hit paste on Mastonaut’s composer, it actually fetches the folder icon from the pasteboard and uploads it as an attachment. I have no idea how to prevent this from happening. Example:
I ended up adding a sanity check: Fetch file URLs if they point to an image, otherwise fetch any image from the pasteboard, unless the pasteboard also contains objects of pasteboard type “fileURL”. Seems to work well enough.