FileOpenPicker class

public sealed class FileOpenPicker : Object

if (rootPage.EnsureUnsnapped())
{
    FileOpenPicker openPicker = new FileOpenPicker();
    openPicker.ViewMode = PickerViewMode.Thumbnail;
    openPicker.SuggestedStartLocation = PickerLocationId.PicturesLibrary;
    openPicker.FileTypeFilter.Add(".jpg");
    openPicker.FileTypeFilter.Add(".jpeg");
    openPicker.FileTypeFilter.Add(".png");

    StorageFile file = await openPicker.PickSingleFileAsync();
    if (file != null)
    {
        // Application now has read/write access to the picked file
        OutputTextBlock.Text = "Picked photo: " + file.Name;
    }
    else
    {
        OutputTextBlock.Text = "Operation cancelled.";
    }
}

internal bool EnsureUnsnapped()
{
    // FilePicker APIs will not work if the application is in a snapped state.
    // If an app wants to show a FilePicker while snapped, it must attempt to unsnap first
    bool unsnapped = ((ApplicationView.Value != ApplicationViewState.Snapped) || ApplicationView.TryUnsnap());
    if (!unsnapped)
    {
        NotifyUser("Cannot unsnap the sample.", NotifyType.StatusMessage);
    }

    return unsnapped;
}

IReadOnlyList<StorageFile> files = await openPicker.PickMultipleFilesAsync();
if (files.Count > 0)
{
    StringBuilder output = new StringBuilder("Picked files:\n");
    // Application now has read/write access to the picked file(s)
    foreach (StorageFile file in files)
    {
        output.Append(file.Name + "\n");
    }
    OutputTextBlock.Text = output.ToString();
}
else
{
    OutputTextBlock.Text = "Operation cancelled.";
}