Prefer attachment_filename for mime type guessing in send_file

In send_file you can explicitly set the filename using attachment_filename, but when attempting to guess the mimetype previously the code would prefer the real filename, this reverses that preference.

Our use case is that the real filename on disk is just a hash, and so the mime type cant be guessed anyway. You can of course explicitly set the mime type as well, but imo if a filename is explicitly passed in that should take precedence