I can't think of a way to remove specific attachments.. A filter can forward a message, and you can use macros to use the original headers and text in the forwarded message, but I don't think you can make it include or exclude specific attachments (it's all or nothing).

You can run an external utility on the original message though. So, if you have (or could make) a simple command-line program that strips the attachments, that might be a solution.

Hopefully someone else has a better idea!