protected function outputContentType($format)
    {
        if (!isset($format)) {
            $contentType = $this->getResponse()->getHeaderLine('Content-Type');
            
            if (empty($contentType)) {
                $format = $this->defaultFormat ?: 'text/html';
            }
        }
        
        if (empty($contentType)) {
            $contentType = $this->getContentType($format);
            $this->setResponseHeader('Content-Type', $contentType);
        }
        
        return $contentType;
    }