+ Open winmail.dat TNEF-encoded Outlook files on iPhone and iPad is the #1 way to view winmail.dat attachment data on your iOS devices.
+ Open ZIP-Archive:
• View the contents of ZIP-Archives
• Extract files from ZIP-Archives
• Export a file to another app

Please note that, there is no need to open the application directly by tapping on application icon on device. ON installation, It get registered as an addon for mail application, you just need to tap on winmail.dat attachment in your email which will the open the application with the content of your attachment.
Here are the Help Instructions:
1) Open up the email and touch on the icon of the winmail.dat file.
2) You will see the "Open in Winmail" screen using a short tap; if not then keep touching the icon till the whole box darkens.
3) Once you keep touching the icon (on left side) itself, after a couple of seconds, a 2-choice menu pops up from the bottom.
4) Click on - "Open in Winmail" app. The winmail.dat file will then open, showing the attachments inside.