@Nullable
    public static String getFileExtension(@NotNull String fileName) {
        @Nullable
        final String fileExt = FilenameUtils.getExtension(fileName);
        return StringUtils.isNoneEmpty(fileExt) ? StringUtils.lowerCase(fileExt) : null;
    }
@Test
    public void testGetFileExtension() {
        String[] fileName = {"something-0.9.5.jar", "lib2-1.1.js", "dir.tmp/noext"};
        String[] expResult = {"jar", "js", null};

        for (int i = 0; i < fileName.length; i++) {
            String result = FileUtils.getFileExtension(fileName[i]);
            assertEquals("Failed extraction on \"" + fileName[i] + "\".", expResult[i], result);
        }
    }