bad example. validation configuration using FormEvent

FileUploadType.php

<?php

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\Form\FormError;
use Symfony\Component\Form\FormEvent;
use Symfony\Component\Form\FormEvents;

class FileUploadType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $basePath = $options['base_path'];

        $builder
            ->add('folder')
            ->add('file', 'file')
            ->addEventListener(FormEvents::POST_SUBMIT, function (FormEvent $event) use ($basePath)
              {
                  /** @var \Symfony\Component\Form\FormInterface $form */
                  $form = $event->getForm();
                  $data = $event->getData();
                  if (file_exists($bathPath.'/'.$data['folder'].'/'.$data['file'])) {
                      $form['file']->addError(new FormError('ファイル名は既に使われています。'));
                  }
              })
        ;
    }

    public function getName()
    {
        return 'file_upload';
    }
}