File Picker not working

Hello i'm using acr-xamarin-forms to allow user to select file in my app. I'm using following code

var device = Resolver.Resolve ();
var mediaPicker = DependencyService.Get ();

if (mediaPicker == null) {
mediaPicker = device.MediaPicker;
}

try {
MediaFile result = await mediaPicker.TakePhotoAsync (
new CameraMediaStorageOptions {
DefaultCamera = CameraDevice.Rear,
});

} catch (Exception ec) {
Debug.WriteLine (ec.Message);

}

And it's working proper in all android and ios device instead of iPhone 4s.

Can any one suggest solution for it.