ImageHandler.php
protected function getFilename($filename)
    {
        $callback = $this->fileCallback;

        if (null === $callback || substr($filename, 0, 1) == '/') {
            return $filename;
        }

        return $callback($filename);
    }
protected function getFilename($filename)
    {
        $callback = $this->fileCallback;

        if (null === $callback || substr($filename, 0, 1) == '/') {
            return $filename;
        }

        return $callback($filename);
    }
When processing the filename, call the callback.