I don't think we want this here. There are other places in the Zotero code/ecosystem where we can try to determine the type (e.g., after we have the file, potentially after we get smarter), and I don't think it's this function's place to assign application/octet-stream. If we don't know what the file is, saving '' to the database is better than application/octet stream, which doesn't really help us and just takes up space.