Have you looked at
"Java Web Services in a Nutshell" Sample: Chapter 3 SAAJ (PDF) starting at "SOAP Messages and MIME Headers" (p.128) yet?
SAAJ is independent of JAX-WS. JAX-WS uses parts of SAAJ.
SAAJ handles attachments through the older "SOAP with Attachments" (SwA) standard.
In JAX-WS attachments are handled through the more recent "SOAP Message Transmission Optimization Mechanism (MTOM)" standard.
JAX-WS: Binary Attachments (MTOM) [ December 11, 2008: Message edited by: Peer Reynders ]