Compelling problem. I don't completely follow you here, are you attaching to the email or streaming content to it?
You are using integrator right? That should allow you harness any utility you want outside of the flow to achieve your objective. If it is a true attachment writing the file to $LAWDIR/bpm/emailattachments should work. If the file name is dynamic it complicates matters, but you can initially hardcode a filename to see if it works. Then it is up to your email client to manage the attachment.