I found out that eventual attachments are moved inline when there are jaxws handlers in place.

In order to prevent this, calls to XOPContent.visitAndRestoreXOPData() were added in the HandlerDelegateJAXRPC, so that the model transits to the xml-valid and the xop:Include are restored:

boolean status = (handlerChain != null ? handlerChain.handleResponse(msgContext) : true);
if (type == HandlerType.ENDPOINT)
XOPContext.visitAndRestoreXOPData();
return status;

This should be done for JAXWS too.