Notes unknown file type as att1.unk

When an e-mail came to our Notes from Exchange Server 5.5 by using Outlook, I see the attachment as att1.unk. I get some info from the Microsoft technet site, Lotus.com, Notes.net. I have upgraded our Domino and Notes 4.6.7 but problem still exist.I have controlled SMTPTBLS.NSF lastly and I could not see any problem in it. Can you help me to solve this problem? Thank you.