in the JS triggerscript I'd try to fetch the ScriptAttachmentBean via the ScriptIssueBean (or ScriptIssueDeltaBean depending on your trigger context).
The ScriptAttachmentBean offers a getContent() method, that seems to represent the actual attachment file.
The rest depends on your email setup.
Did Matthias suggestion help?
This feature is not something built into the core product, and would definitely require the use of Trigger API calls to achieve. I do know that some customers have implemented this behavior.
Services could certainly do this sort of functionality for you.
I'm using the stanard function MKS uses in their triggers Packages.javax.mail. My first problem was I was using getAttachmentBeans instead of getAddedAttachmentBeans. So I'm able to get the attachment, save it to disk through FileOutputStream and then send it.
So it'd be nice to save to disk part but I don't know if that's possible.