how to upload files using retrofit?

i have a web service controller calss like this

public class User {
        private String user; // JSON string
        private CommonsMultipartFile photo1;
        private CommonsMultipartFile photo2;
        private CommonsMultipartFile photo3;
        private CommonsMultipartFile photo4;

        public User(String user, CommonsMultipartFile photo1, CommonsMultipartFile photo2, CommonsMultipartFile photo3,CommonsMultipartFile photo4) { 
            this.user = user; this.photo1 = photo1; this.photo2 = photo2; 
            this.photo3 = photo3; this.photo4 = photo4;}
        public String getUser() { return user; }
    }


    @RequestMapping(value = "/create",method = RequestMethod.POST, produces = "application/json")
    public ResponseEntity<String> create(User item) throws Exception {
        //remaining code

    }

the above create() method accepts object of type User.

In client app i am calling this webservice like bellow using retrofit

@Multipart
    @POST("/create")
    void createUser(@Part("user") User user,
                               @PartMap Map<String,TypedFile> files,
                               Callback<Customer> callback);

But it is giving me no defaut constructor found at controller class in server side. Please check where i am doing wrong