"Winmail.dat Opener" is a free app to open those pesky 'winmail.dat' attachments. This app is free of charge and fully functional, giving you access to the files without any need for further in-app purchases. The optional in-app-purchase will remove ads.

HOW TO OPEN A WINMAIL.DAT FILE:
You have two options:
1.) Select "Winmail.dat Opener" from the context menu of the winmail.dat attachment in your email client. This option does not work with all Android email apps as some do not allow a fine enough attachment-to-application mapping. If there is no "Winmail.dat Opener" in the context menu you have to use the second option.
2.) Save the winmail.dat attachment somewhere on your phone and start the "Winmail.dat Opener" app. The built-in file dialog will open and you can then select the previously saved winmail.dat file.

Once the ‘contained files’ list is displayed, you can open a file in a viewer by tapping it. If you want to save the extracted file you can long tap it.

WHY DOES THE APP REQUIRE A PERMISSION TO CONNECT TO THE INTERNET?
This app will extract the winmail.dat on your phone/tablet. Ordinarily, the extraction process wouldn’t require internet connection but as our app is ad supported, it will ask for permission for internet access.

THE APP CAN'T OPEN MY .DAT FILE:
Kindly note that not every file ending with ".dat" is a winmail.dat file. There are quite a lot of file formats which also use the .dat extension. So, before submitting a one-star review stating that our app didn’t work, please ensure that your file is indeed a winmail.dat (TNEF) file.

THERE ARE NO FILES IN THE WINMAIL.DAT FILE:
Microsoft Outlook does not only store attachments into a winmail.dat file, it also sometimes saves style and formatting informations. In those cases you will get an winmail.dat file which does not contain any additional files. If the winmail.dat file is rather small (less than 20KB) it is rather likely that it contains no files.
From version Winmail.dat Opener 1.10:
Bug fixes
Variant
Arch
Version
DPI
(Android 4.1+)
Latest: 1.3 on October 20, 2016
(Android 4.4+)
Latest: 1.10 on September 30, 2018
All versions
September 30, 2018
September 25, 2018
October 20, 2016