For the time being (AKA as long as we are saving Blobs to DeviceStorage and don't have the download manager yet), we need a MIME filter here.  Since download manager is coming soon, I'd just copy and paste the contents of gaia/shared/js/mime_mapper.js into here and use that.  So for attachments, we use the MIME filter and don't provide the file.  The user should (sadly), still be able to see the file name and be sad that we don't support their attachment.

I don't think this is an issue for related parts, since we only make it a related part if it's an image in the first place.