FileUriUnique.php

<?php

namespace Drupal\file\Plugin\Validation\Constraint;

use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Validation\Attribute\Constraint;
use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldValueValidator;

/**
 * Supports validating file URIs.
 */
class FileUriUnique extends UniqueFieldConstraint {
    public $message = 'The file %value already exists. Enter a unique file URI.';
    
    /**
     * This constraint is case-sensitive.
     *
     * For example "public://foo.txt" and "public://FOO.txt" are treated as
     * different values, and can co-exist.
     *
     * @var bool
     */
    public $caseSensitive = TRUE;
    
    /**
     * {@inheritdoc}
     */
    public function validatedBy() : string {
        return UniqueFieldValueValidator::class;
    }

}