private function _encoded_filename()
    {
        $ext = strtolower(pathinfo($this->source_file, PATHINFO_EXTENSION));
        $encode = md5($this->source_file . http_build_query($this->url_params));
        
        // Build the parts of the filename
        $encoded_name = $encode . '-' . $this->source_modified . '.' . $ext;
        
        return $encoded_name;
    }