public function asMime()
    {
        $part = 'Content-Disposition: form-data; ';
        $part .= 'name="' . $this->key . '"; ';
        $part .= 'filename="' . $this->filename . '"';
        $part .= "\r\nContent-Type: " . $this->deduceMimeType();
        $part .= "\r\n\r\n" . $this->content;

        return $part;
    }