jetbrick-commons/src/main/java/jetbrick/io/FilenameUtils.java
public static String getFileExtension(final String path) {
        if (path == null) {
            return null;
        }
        int extIndex = path.lastIndexOf('.');
        if (extIndex == -1) {
            return null;
        }
        int folderIndex = getFileSeparatorIndex(path);
        if (folderIndex > extIndex) {
            return null;
        }
        return path.substring(extIndex + 1);
    }
public static String getFileExtension(final String path) {
        if (path == null) {
            return null;
        }
        int extIndex = path.lastIndexOf('.');
        if (extIndex == -1) {
            return null;
        }
        int folderIndex = getFileSeparatorIndex(path);
        if (folderIndex > extIndex) {
            return null;
        }
        return path.substring(extIndex + 1);
    }
得到文件扩展名.