@Nullable
  InputStream readContent(int fileId) throws IOException {
    PersistentFSConnection.ensureIdIsValid(fileId);
    int contentId = connection.getRecords().getContentRecordId(fileId);
    if (contentId == 0) return null;
    return readContentByContentId(contentId);
  }
@Nullable
  InputStream readContent(int fileId)