/**
  * Runs through the css file and searches for image files
  * @since 1.2
  *
  * @param string $file The file path to the CSS file
  * @param string $url  The full live (original) file url of the CSS file
  *
  * @return string
  */
 public function invoke_css_file($file, $url)
 {
     $file_content = file_get_contents($file);
     // if this is a CSS file that is on Google Drive already, do not invoke it again
     if (false !== stripos($url, 'https://googledrive.com')) {
         return $file_content;
     }
     $this->_google_drive_cdn->_is_buffer_css_file = true;
     $this->_google_drive_cdn->_buffer_css_file_path = $file;
     $this->_google_drive_cdn->_buffer_css_file_url = $url;
     $file_content = $this->_google_drive_cdn->buffer($file_content);
     $this->_google_drive_cdn->_is_buffer_css_file = false;
     return $file_content;
 }