There is only one file type that uses the .TNEF extension:
Transport Neutral Encapsulation Format
File generated by Microsoft Exchange email programs such as Outlook; appears as an email attachment for email clients that don't recognize Microsoft's proprietary TNEF format; may cause the text "application/ms-tnef" to appear in the email if the email client does not recognize Microsoft Exchange formatting.
NOTE: Transport Neutral Encapsulation Format files with the extension ".tnef" are rare. Instead, unrecognized Exchange messages are usually seen with the filename Winmail.dat (.DAT file).
Updated: September 1, 2011