add tests for getFormat method

app/src/main/java/hexlet/code/Differ.java

@@ -92,7 +92,7 @@ public static String generate(String filepath1, String filepath2, String format)
         return (diff);
     }
 
-    private static String getFormat(String filePath) {
+    static String getFormat(String filePath) {
         String fileName = Paths.get(filePath).getFileName().toString();
         int dotIndex = fileName.lastIndexOf('.');
         if (dotIndex > 0) {
@@ -101,4 +101,6 @@ private static String getFormat(String filePath) {
         throw new IllegalArgumentException("Cannot determine format: " + filePath);
     }
 
+
+
 }

app/src/test/java/hexlet/code/DifferTest.java

@@ -156,6 +156,21 @@ public void testInvalidJson() throws IOException {
         );
     }
 
+    @Test
+    void testGetFormatMultipleDots() {
+        String filePath = "config.dev.json";
+        String expected = "json";
+        String actual = Differ.getFormat(filePath);
+        assertEquals(expected, actual, "Format should be extracted from last extension");
+    }
+
+    @Test
+    void testGetFormatNoExtension() {
+        String filePath = "config";
+        assertThrows(IllegalArgumentException.class, () -> {
+            Differ.getFormat(filePath);
+        }, "Should throw IllegalArgumentException for file without extension");
+    }
 
     private String normalizeSpaces(String input) {
         return input.replaceAll("\\s+", " ").trim();