code: function(command, group, filename, charset) {
          // --------- 封装header
          var fnLength = Buffer.byteLength(filename, charset)
          var bodyLength = protocol.FDFS_GROUP_NAME_MAX_LEN + fnLength
          var header = protocol.packHeader(command, bodyLength, 0)
  
          // --------- 封装body
          var body = new Buffer(bodyLength)
          // 默认都填充上0
          body.fill(0)
          var groupBL = Buffer.byteLength(group, charset)
          body.write(group, 0, groupBL, charset)
          body.write(filename, protocol.FDFS_GROUP_NAME_MAX_LEN, fnLength, charset)
  
          return Buffer.concat([header, body])
      }
docs: 封装只有fileId的包，下载和删除文件时使用
  @param command
  @param group
  @param filename
  @param charset