docs: @Pure
  	public static File addExtension(File filename, String extension) {
  		if (filename != null && !hasExtension(filename, extension)) {
  			String extent = extension;
  			if (!"".equals(extent) && !extent.startsWith(EXTENSION_SEPARATOR)) { //$NON-NLS-1$
  				extent = EXTENSION_SEPARATOR + extent;
  			}
  			return new File(filename.getParentFile(), filename.getName() + extent);
  		}
  		return filename;
  	}