public function addMultipartParam(string $name, $content, string $filename = null, bool $overwrite = false)
    {
        if (array_key_exists($name, $this->multipart) && !$overwrite) {
            return $this;
        }
        $part = ['name' => $name, 'contents' => $content];
        if (!empty($filename)) {
            $part['filename'] = $filename;
        }
        $this->multipart[] = $part;
        return $this;
    }