如何使用改装将图像从内部存储上载到服务器?

我正在尝试将两张图像从android设备的内部存储上传到HTTP服务器。

    public interface ApiService {
        ...

        @Multipart
        @Headers("Content-Type: application/json")
        @POST("signature/{id}")
        Call<String> sendSignature(
                @Header("Authorization") String authorization,
                @Path("id") String id,
                @Part("descrtipion") RequestBody description,
                @Part MultipartBody.Part file1,
                @Part MultipartBody.Part file2);
    }

...


private void sendSignatures(){
    RequestBody description = RequestBody.create(okhttp3.MultipartBody.FORM, getString(R.string.str_file_description));

    File file2 = getFileStreamPath(SIGNATURE2_PATH);
    RequestBody requestFile2 = RequestBody.create(MediaType.parse("image/png"), file2);
    MultipartBody.Part body2 = MultipartBody.Part.createFormData("sign1", file2.getName(), requestFile2);

    File file = getFileStreamPath(SIGNATURE_PATH);
    RequestBody requestFile = RequestBody.create(MediaType.parse("image/png"), file);
    MultipartBody.Part body = MultipartBody.Part.createFormData("sign2", file.getName(), requestFile);

    Call<String> call = ApiFactory.getService().sendSignature(token, PARAMETER_ID, description, body2, body);
    call.enqueue(new Callback<String>() {
        @Override
        public void onResponse(Call<String> call, Response<String> response) {
            if (response.isSuccessful()) Log.d("myLogs", "Yes: " + response.body());
            else Toast.makeText(MainActivity.this, ErrorUtils.errorMessage(response), Toast.LENGTH_LONG).show();
        }

        @Override
        public void onFailure(Call<String> call, Throwable t) {
            Toast.makeText(MainActivity.this, t.toString(), Toast.LENGTH_LONG).show();
        }
    });
}

但服务器返回错误#500。同时,邮递员的请求成功(#200)。

请帮助我解决这个问题:我在java代码中哪里会出错?