formData[key] = value
	}

	// Create the multipart form.
	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"))

pkg mime/multipart, method (*Writer) CreatePart(textproto.MIMEHeader) (io.Writer, error)
pkg mime/multipart, method (*Writer) FormDataContentType() string
pkg mime/multipart, method (*Writer) WriteField(string, string) error
pkg mime/multipart, type File interface { Close, Read, ReadAt, Seek }
pkg mime/multipart, type File interface, Close() error
pkg mime/multipart, type File interface, Read([]uint8) (int, error)