I would try to rename the file with different extensions such as -
and see if my image software will open it.
Were these files sent to you over AOL?
At work we sometimes receive files with the .dat extension. These are a combination of artwork and text but the desktop software won't open them.
I then ask the customer to resend using another method than through AOL and that fixes the problem.
Or if they have to use AOL, ask them to .zip or stuff the files first and see if that will prevent AOL from messing with the files.