Attachments are usually sent with SwA (
SOAP with attachments) where the attachment is sent outside of the SOAP envelope (both are wrapped together with MIME). See for example:
SOAP attachments with JAX-RPC. You may however want to
test your particular SOAP stack with attachments of that particular size. If you run into trouble you may need to find one that supports your needs or switch to an alternate tactic where the service releases a URI from which the file can be downloaded with a more appropriate protocol (with resumable downloads etc.) for a limited amount of time.