apollo-client/src/main/java/com/ctrip/framework/apollo/spi/DefaultConfigFactory.java/DefaultConfigFactory/determineFileFormat
class DefaultConfigFactory:  ConfigFileFormat determineFileFormat(String namespaceName) {
    String lowerCase = namespaceName.toLowerCase();
    for (ConfigFileFormat format : ConfigFileFormat.values()) {
      if (lowerCase.endsWith("." + format.getValue())) {
        return format;
      }
    }

    return ConfigFileFormat.Properties;
  }