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\Attribute\HasNamedArguments;
use Symfony\Component\Validator\Constraint as SymfonyConstraint;

/**
 * File extension constraint.
 */
class FileExtensionConstraint extends SymfonyConstraint {
  
  /**
   * The allowed file extensions.
   *
   * @var string
   */
  public string $extensions;
  public function __construct(mixed $options = NULL, ?string $extensions = NULL, public string $message = 'Only files with the following extensions are allowed: %files-allowed.', ?array $groups = NULL, mixed $payload = NULL) {
    parent::__construct($options, $groups, $payload);
    $this->extensions = $extensions ?? $this->extensions;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getDefaultOption() : ?string {
    return 'extensions';
  }

}