public static String fileExt(String fname) {
        if (isBlank(fname) || fname.indexOf('.') == -1) {
            return null;
        }
        return fname.substring(fname.lastIndexOf('.') + 1);
    }
@Test
    public void testFileExt() {
        String ext = StringKit.fileExt("a.png");
        Assert.assertEquals("png", ext);
    }