官方API文档在此，不过构造请求很坑，其中HttpDo方法的具体实现在此：

func MaterialUploadByData(data []byte, fillType, accessToken, filename string) (res *MaterialResponse) {
    res = &MaterialResponse{}
    contentType := ""
    filenameToLower := strings.ToLower(filename)
    if strings.HasSuffix(filenameToLower, ".jpg") {
        contentType = "image/jpg"
    } else if strings.HasSuffix(filenameToLower, ".png") {
        contentType = "image/png"
    } else if strings.HasSuffix(filenameToLower, ".bmp") {
        contentType = "image/bmp"
    } else if strings.HasSuffix(filenameToLower, ".amr") {
        contentType = "voice/amr"
    } else if strings.HasSuffix(filenameToLower, ".mp4") {
        contentType = "video/mp4"
    } else {
        contentType = "application/octet-stream"
    }
    QYURL := fmt.Sprintf("https://qyapi.weixin.qq.com/cgi-bin/media/upload?access_token=%s&type=%s", accessToken, fillType)
    BOUNDARY := strconv.FormatInt(time.Now().Unix(), 10)

    bodyBuf := bytes.NewBuffer(nil)
    bodyBuf.WriteString(fmt.Sprintf("--%s\r\nContent-Disposition: form-data; name=\"media\";filelength=%s; filename=\"%s\"\r\n"+
        "Content-Type: %s\r\n\r\n",
        BOUNDARY, strconv.Itoa(len(data)), filename, contentType))
    bodyBuf.Write(data)
    bodyBuf.WriteString(fmt.Sprintf("\r\n--%s--\r\n", BOUNDARY))
    req, err := http.NewRequest("POST", QYURL, bodyBuf)
    if err != nil {
        log.Errorf("err = %s", err.Error())
        res.Errcode = -1
        res.Errmsg = err.Error()
        return res
    }
    req.Header.Add("Content-Type", `multipart/form-data; boundary=`+BOUNDARY)
    //req.Header.Add("Content-Length", strconv.Itoa(bodyBuf.Len())) //可以省略
    resStr, err := httputils.HttpDo(req)
    if err != nil {
        log.Errorf("err = %s", err.Error())
        res.Errcode = -1
        res.Errmsg = err.Error()
        return
    }
    err = json.Unmarshal(resStr, res)
    if err != nil {
        log.Errorf("获取mediaId出错，err = %s", err.Error())
        res.Errcode = -1
        res.Errmsg = err.Error()
        return
    }
    return
}
type MaterialResponse struct {
    Errcode   int32  `json:"errcode"`
    Errmsg    string `json:"errmsg"`
    MediaId   string `json:"media_id"`
    FillType  string `json:"type"` //分别有图片（image）、语音（voice）、视频（video），普通文件（file）
    CreatedAt string `json:"created_at"`
    Url       string `json:"url"`
}