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

	// Set the normal formData
	for k, v := range formData {
		w.WriteField(k, v)
	}
	// Set the File formData
	writer, err := w.CreateFormFile("file", "upload.txt")
	if err != nil {
		// return nil, err
		return nil, err
	}
	writer.Write([]byte("hello world"))
	// Close before creating the new request.
	w.Close()

pkg mime/multipart, method (*Writer) Boundary() string
pkg mime/multipart, method (*Writer) Close() error
pkg mime/multipart, method (*Writer) CreateFormField(string) (io.Writer, error)
pkg mime/multipart, method (*Writer) CreateFormFile(string, string) (io.Writer, error)
pkg mime/multipart, method (*Writer) CreatePart(textproto.MIMEHeader) (io.Writer, error)
pkg mime/multipart, method (*Writer) FormDataContentType() string