I'm on a 620 system up to basis patch 60.
When an inbound email contains an attachment with extension .FAX the attachment disappears!!
Any other extensions are fine. The .FAX files in my case are actually .TIF files but the software creates them with .FAX extension.
If I rename the file to have a .FOX extension for example, the attachment makes it into SAP just fine, as does any other extensions I've tried (.TXT, .DOC, .ZIP, .PDF, .XLS etc)
This happens whether I send to a normal SAP users inbox or whether I handle the inbound request using my own SO50 configured class.
We've turned on the Inbound and General trace in SCOT and there is nothing too exciting. The log does show there is a .FAX attachment present (so it's not stripped by the external mail system) but it doesn't indicate that it's been removed.
Please note!! This is NOT inbound Fax handling. It's just a mail that happens to have an attachment which ends with the extension .FAX.