Winmail.dat Opener is a free Android app to open those annoying 'winmail.dat' attachments.
This app is completely free of charge (ad supported) and fully functional, giving you access to your files without the need for further in-app purchases.
The app requires only about 3MB space and only the absolute minimum of permissions.
Your files are processed on your phone and not sent to any external servers. This ensures that your information stays private and the app will still work when you are offline.