There seems to be some issue with the way IMAP in Gmail handles attachments sent via Tiger's Mail.app -- they often appear as generic "mime-attachments" with no apparent way to decode them on the receiving end. While I'm not sure of where exactly the problem is being introduced, I have found a way to rescue these attachments.

The key comes from this hint. However, trying to use openssl on the mime-attachment will fail until you remove the headers. You need to delete the parts above the encoded data that look like this:

--Apple-Mail-3-1071339734
Content-Transfer-Encoding: base64
Content-Type: application/applefile;
	name=file.doc
Content-Disposition: attachment;
	filename="file.doc"

Take note of the filename, and then use it in the command referenced in the earlier hint like this:

openssl base64 -d -in mime-attachment -out file.doc

[robg adds: I haven't tested this one.]