.MS-TNEF File Extension
File TypeMS Transport Neutral Encapsulation Format
What is an MS-TNEF file?
File sometimes generated by Microsoft email programs such as Outlook; saves the message in Microsoft's proprietary TNEF format, which often is not recognized by other email programs; may cause the email to appear with the "application/ms-tnef" text, which is the MIME type for Microsoft Exchange email formatting.
NOTE: MS-TNEF files are rarely seen with the ".ms-tnef" extension. Instead, they are usually seen with the filename Winmail.dat (.DAT file), which includes the MS-TNEF encoded data.