As pointed by Christin, you may try that tutorial. But by default, REST-SOAP-REST conversion with attachments works only as SWA ( Soap with attachment). Hope you are aware that MTOM is backward compatible with SWA. So even if your SOAP service is expecting MTOM and you send as SWA it will work without any issues.
If you would like to send the attachment as MTOM then you can leverage some node js examples available online.
can we get a sample java script to extract the JSON payload which is coming along with multipart form data and convert it to SOAP message with either SWA or MTOM.