PathProcessorFiles.php

<?php

namespace Drupal\system\PathProcessor;

use Drupal\Core\PathProcessor\InboundPathProcessorInterface;
use Symfony\Component\HttpFoundation\Request;

/**
 * Defines a path processor to rewrite file URLs.
 *
 * As the route system does not allow arbitrary amount of parameters convert
 * the file path to a query parameter on the request.
 */
class PathProcessorFiles implements InboundPathProcessorInterface {
    
    /**
     * {@inheritdoc}
     */
    public function processInbound($path, Request $request) {
        if (strpos($path, '/system/files/') === 0 && !$request->query
            ->has('file')) {
            $file_path = preg_replace('|^\\/system\\/files\\/|', '', $path);
            $request->query
                ->set('file', $file_path);
            return '/system/files';
        }
        return $path;
    }

}