code: function getContentType(url: string): string {
    url = trimUrlQuery(url);
    const dot = url.lastIndexOf(".");
    if (dot >= 0) {
      const name = url.substring(dot + 1);
      if (name in contentMap) {
        return contentMap[name];
      }
    }
    return "text/plain";
  }
docs: Returns the content type for the specified URL.  If no specific
  content type can be determined, "text/plain" is returned.
  
  @return String
  The content type.