FilenameFilter to show list files with certain extension and directories

I am using FilenameFilter class to list all files with certain extension in the directory. But I would also like to list all the directories in that directory. I searched through the internet and found out only how to use FilenameFilter for multiple extensions. But how can I do that also with directories?

Here is code that I use to list all .txt files:

File[] listOfFiles = folder.listFiles(new FilenameFilter() {
        public boolean accept(File dir, String name) {
            return name.toLowerCase().endsWith(".txt");
        }
    }); 

Is there even a way to do it via filter or do I have to do it in some other way?