How to consume multipart/mixed payload as multipart?

Question

I have hit a REST API which consumes multipart/mixed data with a payload but the REST API takes it as a string. If I try other Object as Multipart, RequestPart then it throws a 415 UnsupportedMediaType Exception.

This is the multipart/mixed payload which I have passed:

POST /users/jsonform HTTP/1.1
Content-Type: multipart/mixed; boundary=----------boundary
Content-Length: 963

----------boundary
Content-Length: 337
Content-Type: application/json
content-id: 1

GET http://localhost:9090/users/user/1

----------boundary
Content-Length: 353
Content-Type: application/json
content-id: 2
content-transfer-encoding: binary

POST http://localhost:9090/users/addUser

{
    "firstName": "Test",
    "lastName": "Test"
}

----------boundary--
@POST 
@Path("/jsonform") 
@Consumes("multipart/mixed") 
public void simpler(String inputs) throws MessagingException, IOException {
    System.out.println(inputs);
}

If I pass a string argument it reads the payload as a string but If I pass anything other than string, then its throws an error.

I am getting the error:

HTTP Status [415] – [Unsupported Media Type]