apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/util/ConfigFileUtils.java/ConfigFileUtils/getFormat
class ConfigFileUtils:  public static String getFormat(final String originalFilename) {
    final List<String> fileNameSplit = Splitter.on(".").splitToList(originalFilename);
    if (fileNameSplit.size() <= 1) {
      throw new BadRequestException("The file format is invalid.");
    }
    return fileNameSplit.get(fileNameSplit.size() - 1);
  }