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

    public function buildView(FormView $view, FormInterface $form, array $options)
    {
        if ($options['multiple']) {
            $view->vars['full_name'] .= '[]';
            $view->vars['attr']['multiple'] = 'multiple';
        }

        $view->vars = array_replace($view->vars, array(
            'type' => 'file',
            'value' => '',
        ));
    }

 /**
  * {@inheritdoc}
  */
 public function buildView(FormView $view, FormInterface $form, array $options)
 {
     parent::buildView($view, $form, $options);
     $view->vars['filter'] = $options['filter'];
     $view->vars['object'] = $form->getParent()->getData();
     $view->vars['preview'] = $options['preview'];
 }