public function contentType(string $contentType): HttpRequest
    {
        $contentType = trim($contentType);

        // If this is a multipart request and boundary was not defined, we define a boundary as this is required for multipart requests:
        if (stripos($contentType, "multipart/") !== false) {
            if (stripos($contentType, "boundary") === false) {
                $contentType .= "; boundary=\"" . uniqid(time()) . "\"";
                $contentType = preg_replace('/(.)(;{2,})/', '$1;', $contentType); // remove double semi-colon, except after scheme
            }
        }

        return $this->header("Content-Type", $contentType);
    }