These attachments are sent from users of a Microsoft email system. The att1.unk file is received by Notes if the original message was transmitted with MIME encoding. The winmail.dat file is received if the Microsoft email system UUEncodes the message.
Here are some links for more information:
http://office.microsoft.com/assistance/2000/Olmsgfrmtprobs.aspx
http://support.microsoft.com/support/kb/articles/q136/2/04.asp
If you can get the files into a .tnf file extension, you can use the fentun "TNEF-unpacker" from:
http://www.fentun.com