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;
  }

}