Save.cs

public async void SaveThumbnail(StorageFile videofile, string thumbnailfilename)
        {
            var thumbnail = await videofile.GetThumbnailAsync(ThumbnailMode.VideosView);

            var saveFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(thumbnailfilename, CreationCollisionOption.GenerateUniqueName);
            var inputBuffer = new Windows.Storage.Streams.Buffer(2048);

            using (var destFileStream = await saveFile.OpenAsync(FileAccessMode.ReadWrite))
            {
                IBuffer buf;
                while ((buf = (await thumbnail.ReadAsync(inputBuffer, inputBuffer.Capacity, InputStreamOptions.None))).Length > 0)
                    await destFileStream.WriteAsync(buf);
            }
        }