Hi Sandeep,
If you are using the HTTP_AAE adapter, you shouldn't have to use Java mapping to create multi-part content. It should be done by the adapter configuration. I provided the link to check if you have configured the Content-Type in channel. Can you share a screenshot of channel?
Also, you can use SOAPUI to test your configuration. Host a REST service on the local SOAPUI system and provide the URL in your SAP PI system and see if you can get the payload. Alternatively, you can use XPI inspector to further investigate. Unfortunately, at the moment I am out of ideas. Hope someone else can provide more information.
Cheers,
Sanjeev