Unsupported mime-types are strictly a front-end issue.  It's a Gaia issue that it doesn't have file management, not a GELAM issue, so we don't address it in GELAM.  This situation is addressed in message_reader.js where it uses shared/js/mime_mapper to determine based on mime-type and extension whether we can/should download things.

We might also potentially use this flag in the weird cases like Thunderbird where it re-writes a message, detaching the attachment.  In that case it knows the filename, but may no longer have access to the file.  (Thunderbird re-writes the attachment with a reference to the locally stored file, but in the case of IMAP that file may not exist, etc.)