private void OnUploadCompleted(object sender, UploadFileCompletedEventArgs e)
        {
            var uploadablePicture =
                UploadablePictures.FirstOrDefault(elem => elem.PicturePath == e.UploadedFileName);

            if (uploadablePicture != null)
            {
                uploadablePicture.IsUploaded = true;
            }
        }
        private void OnUploadStarted(object sender, UploadFileStartedEventArgs e)
        {
            if (UploadablePictures == null || UploadablePictures.Count == 0)
            {
                return;
            }

            var uploadablePicture =
                UploadablePictures.FirstOrDefault(elem => elem.PicturePath == e.UploadedFileName);

            if (uploadablePicture != null)
            {
                _dropBoxCloudStorageManager.UploadProgressChangedCallback = uploadablePicture.OnUploadProgressChanged;
            }
        }