I have numerous clients with this problem, including myself. We have created custom .oft files in Outlook 2010 that contains a header image, specific formatting for lists, colors etc. When sending it, the email is 100% perfect in the outbox but once received - It is somehow being converted to RTF and loses all formatting and images show up as attachments.
This is using a POP/SMTP account, AV software has been removed to test that, computers rebooted, updates applied and remove and reapplied, cache cleared, Outlook started with the /resetnavpane switch etc.
IF I create a new email and set custom attributes like lists, headers etc - It is received perfectly. This just happens on emails created from saved messages or from oft files?