func AddExtensionType(ext, typ string) error {
	if !strings.HasPrefix(ext, ".") {
		return fmt.Errorf("mime: extension %q missing leading dot", ext)
	}
	once.Do(initMime)
	return setExtensionType(ext, typ)
}

func setExtensionType(extension, mimeType string) error {
	justType, param, err := ParseMediaType(mimeType)
	if err != nil {
		return err
	}
	if strings.HasPrefix(mimeType, "text/") && param["charset"] == "" {