Symfony\Component\Form\Extension\Core\Type\FileType::configureOptions PHP Method

    public function configureOptions(OptionsResolver $resolver)
    {
        $dataClass = function (Options $options) {
            return $options['multiple'] ? null : 'Symfony\Component\HttpFoundation\File\File';
        };

        $emptyData = function (Options $options) {
            return $options['multiple'] ? array() : null;
        };

        $resolver->setDefaults(array(
            'compound' => false,
            'data_class' => $dataClass,
            'empty_data' => $emptyData,
            'multiple' => false,
        ));
    }

 public function configureOptions(OptionsResolver $resolver)
 {
     parent::configureOptions($resolver);
     $resolver->setDefaults(array('image' => ''));
 }