code: func OptionalStringParam(r *http.Request, name string) string {
  	if r.Form == nil {
  		r.ParseMultipartForm(32 << 20)
  	}
  	if vs := r.Form[name]; len(vs) == 1 {
  		return vs[0]
  	}
  	return ""
  }
docs: // OptionalStringParam parses and returns an optional parameter
  // from the form body (multipart/form-data encoded).