FileUriUnique.php

<?php

namespace Drupal\file\Plugin\Validation\Constraint;

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

/**
 * Supports validating file URIs.
 */
class FileUriUnique extends SymfonyConstraint {
    public $message = 'The file %value already exists. Enter a unique file URI.';
    
    /**
     * {@inheritdoc}
     */
    public function validatedBy() : string {
        return '\\Drupal\\Core\\Validation\\Plugin\\Validation\\Constraint\\UniqueFieldValueValidator';
    }

}