I also have encountered these same strange responses. They came in the form of a private email to me regarding a post I made to ridecamp. I did not open the attachment (FUN.MP3.pif) since I did not know the sender. I tried to reply to this message to find out from the sender what the file was but the email address came back as invalid.
DON'T OPEN is my only suggestion. I can't find a listing of this on the Norton Anti-virus site but I wouldn't chance it.