Check if mimetype handler exists in delete_related_files signal handler.
Review Request #13512 — Created Jan. 31, 2024 and submitted
File attachments recently gained the ability to delete related files
when the main file is deleted. This can include things like images
deleting thumbnail versions, or Office documents deleting PDF copies.
This new code wasn't checking if the mimetype_handler existed, which
meant if we ended up with an attachment that didn't have a mimetype
specified, it would crash.
Was able to delete file attachments that didn't have the