code: func getMultipartOverhead() int64 {
  	multipartOnce.Do(func() {
  		var b bytes.Buffer
  		w := multipart.NewWriter(&b)
  		part, _ := w.CreateFormFile("0", "0")
  
  		dummyContents := []byte("0")
  		part.Write(dummyContents)
  
  		w.Close()
  		multipartOverhead = int64(b.Len()) - 3 // remove what was added
  	})
  	return multipartOverhead
  }
docs: // multipartOverhead is how many extra bytes mime/multipart's
  // Writer adds around content