如何为上传请求生成Boundary？

const httpRequest = http.createHttp() 
const formData = new FormData() 
 
for (const key in body) { 
  formData.append(key, body[key]); 
} 
 
const fileName = filepath.substring(filepath.lastIndexOf('/') + 1); 
formData.append('file', filepath, fileName) 
try { 
  const response = await httpRequest.request(url, { 
    method: http.RequestMethod.POST, 
    header: headers, 
    extraData: body 
  }) 
 
  if (response.responseCode === http.ResponseCode.OK){ 
    return response.result 
  }else { 
    return Promise.reject({code: response.responseCode, message: response.responseCode + ""}) 
  } 
} catch (e) { 
  return Promise.reject({code: "-1", message: "unknown error"}) 
}