Got it working thanks to jwsdp mailing list... Setting content type to null forces check of getContentType internally from the DataSource..
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//receive is the ws client implementation
Map<String, Object> responseContext = ((javax.xml.ws.BindingProvider)receive).getResponseContext();
HashMap map = (HashMap) responseContext.get(MessageContext.INBOUND_MESSAGE_ATTACHMENTS);
key = key.substring(1, key.length()-1);
//Use the key to get the DataHandler
DataHandler attachments = (DataHandler) att.get(key);
MimeMultipart mmp = new MimeMultipart(attachments.getDataSource(), null);
MimeBodyPart mbp = mmp.getBodyPart(0); etc...
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