TNEF-php parser Transport Neutral Encapsulation Format or TNEF is, despite the name, a proprietary E-mail attachment format used by Microsoft Outlook and Microsoft Exchange Server. An attached file with TNEF encoding is most often named winmail.dat or win.dat, and has a MIME type of Application/MS-TNEF. The official (IANA) media type, however is application/vnd.ms-tnef .