failed test with mock files

Im starting with mock in my unit tests. Wanted to upgrade method to using files. Mocks with some strings or numbers work, but now code has failed test. I have class with method:

public class FileService {
    public List<File> findFilesUsingSuffix(File root, String suffix) {
        List<File> list = new ArrayList<>();
        if (root.isDirectory()) {
            File[] files = root.listFiles();
            if (files != null) {
                for (File file : files) {
                    list.addAll(findFilesUsingSuffix(file, suffix));
                }
            }
        } else if (root.getName().endsWith(suffix)) {
            list.add(root);
        }
        return list;
    }
}

and test:

public class FileServiceTest {
    private FileService fileService;
    private TemporaryFolder temporaryFolder;
 
    @Before
    public void setUp() throws Exception {
        fileService = mock(FileService.class);
 
        temporaryFolder = mock(TemporaryFolder.class);
        File file1 = temporaryFolder.newFile("file.java");
        File file2 = temporaryFolder.newFile("file.swf");
        File file3 = temporaryFolder.newFile("file2.java");
        File file4 = temporaryFolder.newFile("picture.jpg");
        File file5 = temporaryFolder.newFile("photo.jpg");
        File file6 = temporaryFolder.newFile("movie.mp4");
        File file7 = temporaryFolder.newFile("music.mp3");
        File file8 = temporaryFolder.newFile("someFile.java");
        File file9 = temporaryFolder.newFile("someDoc.doc");
        File file10 = temporaryFolder.newFile("excelThing.xls");
        File file11 = temporaryFolder.newFolder("java");
    }
 
    @Test
    public void findFilesUsingSuffixIsTrue() throws IOException {
        //given
        List<File> expectedList = List.of(
                new File("file.java"), new File("file2.java"), new File("someFile.java")
        );
 
        when(fileService.findFilesUsingSuffix(ArgumentMatchers.any(File.class),any(String.class))).thenCallRealMethod();
 
        //when
        List<File> result = fileService.findFilesUsingSuffix(temporaryFolder.getRoot(), ".java");
 
        //then
        assertEquals(expectedList, result);
    }
}

I'm getting empty list in result, but idk what i did incorrectly