recording.go
func (r *Recording) URL(extension string) string {
	if !strings.HasPrefix(extension, ".") {
		extension = "." + extension
	}
	return strings.Join([]string{BaseURL, r.APIVersion, "Accounts", r.AccountSid, recordingsPathPart, r.Sid + extension}, "/")
}
func (r *Recording) URL(extension string) string {
	if !strings.HasPrefix(extension, ".") {
		extension = "." + extension
	}
	return strings.Join([]string{BaseURL, r.APIVersion, "Accounts", r.AccountSid, recordingsPathPart, r.Sid + extension}, "/")
}
// URL returns the URL that can be used to play this recording, based on the
// extension. No error is returned if you provide an invalid extension. As of
// October 2016, the valid values are ".wav" and ".mp3".