code: public static String normalizeExtension(String file, String extension)
      {
          Check.notNull(file);
          Check.notNull(extension);
  
          final int length = file.length() + Constant.DOT.length() + extension.length();
          final StringBuilder builder = new StringBuilder(length).append(removeExtension(file)).append(Constant.DOT);
          if (extension.startsWith(Constant.DOT))
          {
              builder.append(getExtension(extension));
          }
          else
          {
              builder.append(extension);
          }
          return builder.toString();
      }
docs: Normalize the file extension by ensuring it has the required one.
  
  @param file The original file name (must not be <code>null</code>).
  @param extension The desired extension, will replace the other one if has (must not be <code>null</code>).
  @return The normalized file with its extension.
  @throws LionEngineException If invalid arguments.