Adding business object attachments to an outbound notification can be done when sending a notification using the Business Rule notification.
You can attach emails to an outbound notification as long as the notification is sent from the Object business rules Triggered actions.
In this case I am using the Incident IncidentResolutionNotification Business rule to show how to send attachments.
The business rule is fired when the incident.status is set to Resolved
After you set up the notification choose Next
It is on this page that you set up the sending of the attachments. You can prompt the user for attachments or you can include attachments from the selected business object