How to display “winmail.dat” files
Attachments called “winmail.dat” are sent by Microsoft Outlook and may contain calendar events, contacts, nested messages, or notes. With or without Attachment Tamer, Apple Mail cannot display these attachments by iself.
Letter Opener is a third-party plug-in that converts Microsoft Outlook's “winmail.dat” files on the fly, so you can actually see and use what the sender intended you to have. Attachment Tamer and Letter Opener are compatible.