Check out this forum thread:
Hooking in another process or content_filter?
This was my thread originally for doing something similar to your request via postfix. Dux T used the solution (I think he came up with it... read the whole thread for proper credits) to attach a footer to every message.
Basically you are taking the mail message, altering it and injecting it back into the Zimbra process. You would have to write an external script to remove, upload, and link the attachments but it should be fairly trivial as there are mime handling routines in most scripting languages.