Save File Thumbnail as image in windows runtime

I want to get Thumbnail of files stored in Videos folder and save them as image in my local folder . here is my code to get files .

var v = await KnownFolders.VideosLibrary.GetFilesAsync();
foreach (var file in v)
{
    var thumb = await file.GetScaledImageAsThumbnailAsync(Windows.Storage.FileProperties.ThumbnailMode.SingleItem);
    BitmapImage Img = new BitmapImage();
    Img.SetSource(thumb);
    await ApplicationData.Current.LocalFolder.CreateFolderAsync("VideoThumb");
    var imageFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(
              "VideoThumb\\" + file.Name, CreationCollisionOption.FailIfExists);
    var fs = await imageFile.OpenAsync(FileAccessMode.ReadWrite);
    //I don't know how to save thumbnail on this file !
}

my project is a Windows Phone 8.1 Runtime C# app .

There are couple of things you need to handle:

you don't need BitmapImage, as thumbnail provides a stream which you can write directly to file,
you are not handling the case when create file method fails (file exists),
create folder method will throw exception as the folder is created (or already exists) with the first file in foreach. Rather than that, create/open folder outside foreach,
also creating image with file.Name is not a good idea, hence those are videos and their extension will likely be mp4/mpg/other not jpg/png/other,
remember to add capabilities in packageappx.manifest file.

I think the below code should do the job:

private async Task SaveViedoThumbnails()
{
    IBuffer buf;
    StorageFolder videoFolder = await ApplicationData.Current.LocalFolder.CreateFolderAsync("VideoThumb", CreationCollisionOption.OpenIfExists);
    Windows.Storage.Streams.Buffer inputBuffer = new Windows.Storage.Streams.Buffer(1024);

    var v = await KnownFolders.VideosLibrary.GetFilesAsync();
    foreach (var file in v)
    {
        var thumb = await file.GetScaledImageAsThumbnailAsync(Windows.Storage.FileProperties.ThumbnailMode.SingleItem);
        var imageFile = await videoFolder.CreateFileAsync(file.DisplayName + ".jpg", CreationCollisionOption.ReplaceExisting);
        using (var destFileStream = await imageFile.OpenAsync(FileAccessMode.ReadWrite))
            while ((buf = (await thumb.ReadAsync(inputBuffer, inputBuffer.Capacity, Windows.Storage.Streams.InputStreamOptions.None))).Length > 0)
                await destFileStream.WriteAsync(buf);
    }
}