LOTUS Notes Domino 7 file attachments being received as WIN.DAT

I have users with the issue of attaching a file to LOTUS Notes Domino 7 that are sent out as PDF files however when received it comes over as a WIN.DAT file and not a PDF file.

Now it does not happen all of the time meaning the PDF file is attached and goes out as a PDF and received as PDF file.

Any idea of why this is happening?

Help is appreciated,

Rick M.