Binu George

If an attachment is smaller than the size specified in threshold parameter, the runtime will inline the binary data as base64 binary instead of creating an attachment.
Check your threshold configuration and size of attachment. May be helpful. JAX-WS Attachment – Enable MTOM for JAX-WS Web services

RPC style web service uses the names of the method and its parameters to generate XML structures that represent a method’s call stack. While document style indicates that the SOAP body contains a XML document which can be validated against pre-defined XML schema document. A good starting point : SOAP Binding: Difference between Document and RPC Style Web Services