问golang 用http提供下载文件，下载结果错误问题？

func DownloadFile(fileFullPath string, rw http.ResponseWriter) error {
    file, err := os.Open(fileFullPath)
    if err != nil {
        return err
    }
    defer file.Close()

    fileName := path.Base(fileFullPath)
    //fileName = url.QueryEscape(fileName) // 防止中文乱码
    fileName = fileName[strings.LastIndex(fileName, string(filepath.Separator))+1:]
    //rw.Header().Add("Content-Type", "multipart/form-data")
    //rw.Header().Add("Content-Disposition:", "attachment; filename=\""+fileName+"\"")
    rw.Header().Set("Content-Type", "application/octet-stream")
    rw.Header().Set("content-Disposition", fmt.Sprintf("attachment;filename=%s", fileName))
    //rw.Header().Add("Content-Transfer-Encoding", "binary")
    //rw.Header().Add("Expires", "0")
    //rw.Header().Add("Cache-Control", "must-revalidate")
    //rw.Header().Add("Pragma", "public")
    _, error := io.Copy(rw, file)
    if error != nil {
        return err
    }