code: func (material *Material) ImageUpload(filename string) (url string, err error) {
  	var accessToken string
  	accessToken, err = material.GetAccessToken()
  	if err != nil {
  		return
  	}
  
  	uri := fmt.Sprintf("%s?access_token=%s", mediaUploadImageURL, accessToken)
  	var response []byte
  	response, err = util.PostFile("media", filename, uri)
  	if err != nil {
  		return
  	}
  	var image resMediaImage
  	err = json.Unmarshal(response, &image)
  	if err != nil {
  		return
  	}
  	if image.ErrCode != 0 {
  		err = fmt.Errorf("UploadImage error : errcode=%v , errmsg=%v", image.ErrCode, image.ErrMsg)
  		return
  	}
  	url = image.URL
  	return
  
  }
docs: //ImageUpload 图片上传