That email program is probably as old as the Eudora email program I use and have had since my first PC. Eudora still works find for me including the ability to open attachments. Eudora no longer has software support so I can't recommend you switch to it unless you're willing to play with it a bit. It is a free program though.

Other than the suggestions I made earlier, I can only recommend the usual reboot and try again procedure. You could try right-clicking the attachment and selecting "properties". That will tell you what kind of file it is and you can check to see that you do have a program that will open files with that extension.