The best way to do this is:
1. Look at the MIME source of the document by opening the document, then using menu option View / Show / Page Source.
2. Select the entire source (or just the part with the MIME attachment - but you must include the entire MIME part including the content separators).
3. Copy it to the clipboard and paste into Notepad.
4. Save the Notepad document as a text file.
5. If you have installed the Base64 decoder, you should have a context menu item called decode. Right click your new text file and select "decode". You will now have an opportunity to save the decoded file to disk.
Sounds more longwinded than it actually is.
HTH
Chris Linfoot
http://chris-linfoot.net
-- edited by author to include the following
Beware viruses. Many viruses (Klez.H is a prime example) deliberately send broken MIME in order to exploit known weaknesses in certain MUAs. A Klez.H infected email will turn up in the Notes client "missing" an attachment, which can be recovered using the technique described here (unless your AV system has nuked it server side). You _could_ end up with a live virus in your local file system.
When recovering attachments from emails with apparently broken MIME, bear this in mind.