The winmail.dat file encapsulates mail attached files. It is provided by Outlook according your correspondent has written his mail using the RTF format. After receiving such files, proceed the following :
- ask your friend to check his Outlook configuration
- install ytnef to extract attachment from winmail.dat
$ sudo aptitude install ytnef
$ ytnef -f . winmail.dat
Attachments are now available in the current folder.
