TNEF is a program for unpacking Microsoft MIME attachments of type "application/ms-tnef".
It is used to extract the contents of winmail.dat files that are attached to e-mail by Outlook mail clients.
For example, save the attachment as winmail.dat, then:
The attachments inside winmail.dat should be in the tmp folder.
See also: Claws Mail on this wiki for an in-application mail client TNEF decoder plug-in.