".mht" and ".mhtml" attachments are treated in different ways by various email systems.
- Some (Gmail to Fastmail) treat them as
Content-Type: message/rfc822;
This seems to produce an empty display (either previewed in the Fastmail web interface or downloaded and viewed in Internet Explorer).
- Others (Outlook.com sent to Fastmail or Gmail) treat them as
Content-Type: application/octet-stream;
These can be downloaded and viewed correctly using Internet Explorer from either the Fastmail or Gmail web interface.
- Gmail to Outlook.com produces very odd results. These are treated as
Content-Type: message/rfc822;
A 225 KB .mht attachment sent from Gmail to Outlook.com and auto-forwarded to Fastmail results in:- 914 byte placeholder only in Outlook.com
- 34 byte placeholder only in Fastmail after forwarding
- Fastmail to Fastmail produces a proper sized attachment which can be previewed properly using the Fastmail web interface (see screen image capture here) but which is displayed improperly when downloaded and viewed in Internet Explorer.
- Fastmail to Gmail are treated as
Content-Type: message/rfc822;
These can't be previewed (Gmail limitation) but can be downloaded and displayed properly in Internet Explorer.
So unless you use a zip or similar container I think you can't depend on .mht or .mhtml attachments getting properly through email transfer.