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';
    }

}