Try this Rose:
Winmail.dat is usually attached to emails when someone uses Microsoft Outlook. Tell the sender to send emails in plain text format.
I usually ignore it, if the sender wants to send something important he will find a way.
If you really believe you need to decode the file, try this free tool: http://www.winmail-dat.com/online.php
I did already try the TNEF's enough, but I can't seem to get it to work as it is meant to. I have forwarded the link to my workplace which is where the emails usually come from and hopefully they will fix it at their end. The viewer worked though, so that was great. Nice and simple too.