if you're talking about deadlines definitions where one of the types is notification then best way is to define the mail session as server resource and bind it to JNDI under mail/jbpmMailSession. That should be directly picked up notification service and be used to send emails. Alternatively you can define it via property file named email.properties that should be placed on root of the classpath.
For attachments, you should be able to add additional task variable named attachments that defines comma separated list of URLs that will be used to fetch attachments on send time, make sure that the items are valid URL string representation and that should be added to the email message.
I will try JNDI option. And regarding URL what kind of url's are supported, does it support file:// ? the reason I would like to use file protocol is jBPM will be running in embedded mode so I guess that is the best option for me. I am not able to find documentation about this or I might have not looked at proper place.