code: function _parseFileId(body, charset) {
      var group = helpers.trim(body.toString(charset, 0, protocol.FDFS_GROUP_NAME_MAX_LEN))
      var filename = helpers.trim(body.toString(charset, protocol.FDFS_GROUP_NAME_MAX_LEN))
  
      return group + '/' + filename
  }
docs: -------------- helpers 
  parse file id from body
  @param body
  @param charset
  @private