If an attachment has been encoded (by the sender's email software) in a format that your email software cannot decode, it will just show it in the text format in which it was sent.
Standard email only works with plain text, so attachemnts are always encoded into plain text while sending, and usually decoded when received. There are many different encoding methods, with the old standard being UUEncode, there's also BinHex, Mime, AppleSingle, AppleDouble, and others.
As the previous post said you can get the sender to use another encoding method (some email software, eg, Eudora, allows you to choose from a list of encoders).
Alternatively, save the email message to your disk as a plain text file (NOT RTF), and then drag it to Stuffit Expander, which understands most of the encoding methods. It will ignore the email text, and decode the encoded text portions.