You're just getting either one partor a yEnc.

If it's yEnc it should say so in the subject then you would need to download
something to decode it for that. There are some yEnc decoders for OE you can
find via Google.

If it's only part, you should see in the subject line something like (1/16,
2/6, 3/16, etc.)
You would need to download all parts then highlight all the subject lines
then do a Combine and Decode which will be available in the Message menu.
Make sure they are in order (1 - whatever the last number is), then combine
and decode then save the attachment.