public function getContentType()
    {
        if (isset($this->responseInfo['content_type'])) {
            $contentType = $this->responseInfo['content_type'];
        } else {
            $contentType = isset($this->responseHeaders['Content-Type']) ? $this->responseHeaders['Content-Type'] : null;
        }
        
        if ($contentType) {
            $contentTypeParts = preg_split('/\s*[;,]\s*/', $contentType);
            return strtolower($contentTypeParts[0]);
        }
        
        return null;
    }