So I read a bit about this and for a server response, the recommended contentType value for unknown type is `''`. The reasoning being that the client may be able to guess the mime type. But, I figured since Zotero is the client and there will be no more guessing, we can just go forth with octect-stream. This, btw, is only necessary so that the code in translate_item.js does not go into an infinite loop. Though I kind of compensated for a `''` contentType anyway.