Retrofit 2.0 multipart

I have working OkHttp MultiPart request :

multi = new MultipartBody.Builder()
                .setType(MultipartBody.FORM)
                .addFormDataPart("email", m)
                .addFormDataPart("password", p)
                .addFormDataPart("user_name", n)
                .addFormDataPart("user_phone", phone)
                .addFormDataPart("photo", "avatar.jpg", RequestBody.create(MediaType.parse("image/jpg"), imgToArray(bmpChosenPic)))
                //.addFormDataPart("photo", "1")
                .build();
        request = new Request.Builder()
                .url(Utils.TEST_BASE_URL + "" + REG_URL)
                .post(multi)
                .build();
        client.newCall(request).enqueue(new Callback()...

Now I try to make the same function using Retrofit 2.0. Here's the service:

@Multipart
@POST("/api/v1/auth/register")
Call<Registration>  registration (@Part("email") RequestBody email,
                                  @Part("password") RequestBody password,
                                  @Part("user_name") RequestBody userName,
                                  @Part("photo") RequestBody url,
                                  @Part("phone") RequestBody phone);

So according to this interface I try to make a Call :

  RequestBody photoR = RequestBody.create(MediaType.parse("image/jpg"), imgToArray(bmp));
    RequestBody nameR = RequestBody.create(MediaType.parse("text/plain"), name);
    RequestBody emailR = RequestBody.create(MediaType.parse("text/plain"), email);
    RequestBody passR = RequestBody.create(MediaType.parse("text/plain"), pass);
    RequestBody phone = RequestBody.create(MediaType.parse("text/plain"), "");
    RegistaionI service = ServiceGenerator.createService(RegistaionI.class);
    retrofit2.Call<Registration> call = service.registration(
            emailR,
            passR,
            nameR,
            photoR,
            phone);
    call.enqueue(new retrofit2.Callback<Registration>() {
        @Override
        public void onResponse(retrofit2.Call<Registration> call, retrofit2.Response<Registration> response) {
            RegistrationData data = response.body().getData();
            System.out.println(data.getId());
        }

        @Override
        public void onFailure(retrofit2.Call<Registration> call, Throwable t) {

        }
    });

But, unfortunately, I get NPE in this line:

 RegistrationData data = response.body().getData();

So I suppose, that something's wrong in my code. I've read related topics on SOF and at the Retrofit branch on GitHub but didn't find working solution. Please, help.

I also had so much trouble making this kind of request to work. I end up using this to upload an Image or a Video:

@Multipart
@POST(Constants.URL_UPLOAD)
Call<ResponseBody> upload(@PartMap Map<String, RequestBody> params);

And the method API call

private String uploadFile(String path, final String type) throws IOException, JSONException {

    Map<String, RequestBody> map = new HashMap<>();
    map.put("Id", Utils.toRequestBody("0"));
    map.put("Name", Utils.toRequestBody("example"));
    String types = path.substring((path.length() - 3), (path.length()));

    if (path != null) {
        File file2 = new File(path);
        RequestBody fileBody = RequestBody.create(MediaType.parse(type), file2);
        map.put("file\"; filename=\"cobalt." + types + "\"", fileBody);
    }

    Call<ResponseBody> call = cobaltServices.upload(map);
    ResponseBody response = call.execute().body();

    return RESULT_OK;
}