code: func faviconMIMEType(fileURL string) string {
  	switch {
  	case strings.HasSuffix(fileURL, ".png"):
  		return "image/png"
  	case strings.HasSuffix(fileURL, ".ico"):
  		return "image/ico"
  	case strings.HasSuffix(fileURL, ".jpeg"):
  		fallthrough
  	case strings.HasSuffix(fileURL, ".jpg"):
  		return "image/jpeg"
  	case strings.HasSuffix(fileURL, ".gif"):
  		return "image/gif"
  	}
  	return ""
  }
docs: // faviconMIMEType derives the MIME type from a URL's file extension. Only valid
  // favicon image formats are supported.