code: public Map<String, Object> getFileMetaData() {
      if (file == null) {
        file = new HashMap<String, Object>();
      }
      Map<String, Object> meta;
      if (!file.containsKey(FILE_META)) {
        meta = new HashMap<String, Object>();
        meta.put(FILE_SIZE, actualFile.getSize());
      } else {
        meta = (Map<String, Object>) file.get(FILE_META);
      }
      return meta;
    }
docs: 获取文件的metaData
  
  @return