Outlook allows to open attachment, edit it and send it back. I don't agree with this behaviour but users do like it.
The idea of zimlet is to add the edit button (or popup menu item) in reply/send message form. In this case zimlet will open modal window, download attachment to temp folder, start assotiated application. After saving file and closing application user will see window with "replace" and "cancel" buttons. If he press "replace" the old attachment will be replaced with new one.
I do not know zimlets interface and all "APIs". Does this idea could be fulfilled (all necessary functions to download and replace attachment are exist)?