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 {
  public function __construct(public string $extensions, public string $message = 'Only files with the following extensions are allowed: %files-allowed.', ?array $groups = NULL, mixed $payload = NULL) {
    parent::__construct(groups: $groups, payload: $payload);
  }

}