code: func (material *Material) MediaUpload(mediaType MediaType, filename string) (media Media, err error) {
  	var accessToken string
  	accessToken, err = material.GetAccessToken()
  	if err != nil {
  		return
  	}
  
  	uri := fmt.Sprintf("%s?access_token=%s&type=%s", mediaUploadURL, accessToken, mediaType)
  	var response []byte
  	response, err = util.PostFile("media", filename, uri)
  	if err != nil {
  		return
  	}
  	err = json.Unmarshal(response, &media)
  	if err != nil {
  		return
  	}
  	if media.ErrCode != 0 {
  		err = fmt.Errorf("MediaUpload error : errcode=%v , errmsg=%v", media.ErrCode, media.ErrMsg)
  		return
  	}
  	return
  }
docs: //MediaUpload 临时素材上传