FileExtensionConstraint.php

<?php

declare (strict_types=1);
namespace Drupal\file\Plugin\Validation\Constraint;

use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Validation\Attribute\Constraint;
use Symfony\Component\Validator\Constraint as SymfonyConstraint;

/**
 * File extension constraint.
 */
class FileExtensionConstraint extends SymfonyConstraint {
  
  /**
   * The error message.
   *
   * @var string
   */
  public string $message = 'Only files with the following extensions are allowed: %files-allowed.';
  
  /**
   * The allowed file extensions.
   *
   * @var string
   */
  public string $extensions;
  
  /**
   * {@inheritdoc}
   */
  public function getDefaultOption() : ?string {
    return 'extensions';
  }

}