How to stop winmail.dat files
Winmail.dat or TNEF (Transport Neutral Encapsulation Format) files are created by Outlook or Microsoft Exchange. These files are not readable by normal programs, and mail-list® removes them automatically.
So when users have included a valid attachment, by the time it's put onto the Internet, it has been changed to a Winmail.dat file.
Therefore, people are sending attachments, but nobody on the mailing list is receiving them.
For more information see
To adjust an individual's Outlook settings, see
On Office 365, adjust the server settings to resolve this problem for all users