Send multipart/form-data form with GO

post.go

type field struct {
	Key   string `json:"key"`
	Value string `json:"value"`
}

func Post(url string, fields []field) error {

	var b bytes.Buffer
	w := multipart.NewWriter(&b)

	var (
		fw  io.Writer
		err error
	)

	for _, f := range fields {

		if f.Key == "file" {
			fw, err = w.CreateFormFile(f.Key, filepath.Base(f.Value))
			fd, err := os.Open(filepath.Base(f.Value))
			if err != nil {
				log.Println("Open file failed:", err)
				return err
			} else {
				_, err = io.Copy(fw, fd)
				fd.Close()
				if err != nil {
					log.Println("Copy file failed:", err)
					return err
				}
			}
		} else {
			fw, err = w.CreateFormField(f.Key)
			if err != nil {
				return err
			}
			if _, err := fw.Write([]byte(f.Value)); err != nil {
				return err
			}
		}

	}
	w.Close()

	req, err := http.NewRequest("POST", url, &b)
	if err != nil {
		return err
	}
	req.Header.Set("Content-Type", w.FormDataContentType())

	client := &http.Client{}
	res, err := client.Do(req)
	if err != nil {
		return err
	}
	if res.StatusCode != http.StatusOK {
		fmt.Printf("error post: %s\n", res.Status)
		err = fmt.Errorf("bad status: %s", res.Status)
	}
	return nil
}