public function add(string $content, string $mimeType = null, $encoding = null): void
    {
        $contentPart = $this->make();
        $contentPart->content = $content;
        if ($mimeType !== null) {
            $contentPart->mimeType = $mimeType;
        }
        if ($encoding !== null) {
            $contentPart->encoding = $encoding;
        }
        $this->set($contentPart);
    }