private function _extract_file_extension()
    {

        if (empty($this->filename)) return false;

        if (!empty($this->mime_type)) {
            $array                = explode('/', $this->mime_type);
            $this->file_extension = array_pop($array);
        } else {
            if (strpos($this->filename, '.') !== false) {
                $array                = explode('/', $this->mime_type);
                $this->file_extension = array_pop($array);
            } else {
                $this->file_extension = 'unknown';
            }
        }

        return true;
    }