@Multipart
@POST("/files")
Call<UpLoadPictureResponse> uploadPictures(@Part("access_token") RequestBody accessToken, @Part MultipartBody.Part image);

  public static MultipartBody.Part prepareFileAsPart(String name, String fileUrl) {
            File file = new File(fileUrl);
            RequestBody body = RequestBody.create(MediaType.parse("image/jpeg"), file);
            return MultipartBody.Part.createFormData(name, file.getName(), body);
        }

  service.uploadPictures(RequestBody.create(null, accessToken), prepareFileAsPart("file", url)).enqueue()...