mystuff我有一個處理發送電子郵件的包。package mystuffimport (    "crypto/tls"    "gopkg.in/gomail.v2")type Params struct {    From, To, Subject, Body, Filename string}func Mail(p Params) (err error) {    m := gomail.NewMessage()    m.SetHeader("From", p.From)    m.SetHeader("To", p.To)    m.SetHeader("Subject", p.Subject)    m.SetBody("text/plain", p.Body)    m.Attach(p.Filename)    d := gomail.Dialer{Host: "smtp.example.com", Port: 25}    d.TLSConfig = &tls.Config{InsecureSkipVerify: true}    if err := d.DialAndSend(m); err != nil {        panic(err)    }    return}和發送電子郵件的主要部分package mainimport . "mystuff"func main() {    Mail(Params{From: "[email protected]", To: "[email protected]", Subject: "Sub", Body: "B", Filename: "/tmp/image.jpg" })}我想作為Filename一個可選參數。例如    Mail(Params{From: "[email protected]", To: "[email protected]", Subject: "Sub", Body: "B" })}