winmaildat.com is described as 'Convert winmail.dat or ATT0001.dat files and extract the contained files into accessible, readable files'. There are five alternatives to winmaildat.com, not only websites but also apps for Windows and Mac. The best alternative is Winmail.dat Viewer Pro Edition. It's not free, so if you're looking for a free alternative, you could try Wabmail or Winmail.dat Reader. Other great sites and apps similar to winmaildat.com are Letter Opener (Free) and Winmail Opener (Free).
Read, view, open, convert and save the files on winmail.dat email attachments. Easy-to-use. Graphic interface (no command-line tool). Often you can obtain winmail.dat attachment in gmail and then you need this is tool.