diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 65508a6..0bf0278 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,10 +14,10 @@ jobs:
         python-version: ["3.10", "3.11", "3.12"]
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
 
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@v5
         with:
           python-version: ${{ matrix.python-version }}
 
diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index b2d572b..a4f94ad 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -21,10 +21,10 @@ jobs:
       id-token: write
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v4
 
       - name: Set up Python 3.11
-        uses: actions/setup-python@v6
+        uses: actions/setup-python@v5
         with:
           python-version: "3.11"
 
diff --git a/tests/test_diff.py b/tests/test_diff.py
index a5a096c..79ea278 100644
--- a/tests/test_diff.py
+++ b/tests/test_diff.py
@@ -105,6 +105,24 @@ def test_three_envs(self):
         assert results["prod"].count == 1
 
 
+class TestChangeStr:
+    """Tests for Change.__str__()."""
+
+    def test_str_added(self):
+        c = Change(key="port", change_type=ChangeType.ADDED, new_value=8080, env="prod")
+        expected = "[+] port = 8080  (env: prod)"
+        assert str(c) == expected
+
+    def test_str_removed(self):
+        c = Change(key="port", change_type=ChangeType.REMOVED, old_value=8080, env="dev")
+        expected = "[-] port (was 8080)  (env: dev)"
+        assert str(c) == expected
+
+    def test_str_changed(self):
+        c = Change(key="host", change_type=ChangeType.CHANGED, old_value="localhost", new_value="prod.example.com", env="prod")
+        assert str(c) == "[~] host: 'localhost' \u2192 'prod.example.com'  (env: prod)"
+
+
 class TestDiffResultHelpers:
     def test_by_type(self):
         changes = [
diff --git a/tests/test_loader.py b/tests/test_loader.py
index 9bab060..70114a8 100644
--- a/tests/test_loader.py
+++ b/tests/test_loader.py
@@ -85,6 +85,60 @@ def test_ignores_comments(self):
         assert "This is a comment" not in result
 
 
+class TestLoadUnsupported:
+    """Tests for fallback behaviour with unknown file extensions."""
+
+    def test_fallback_yaml_content_in_cfg(self):
+        """Files with .cfg containing valid YAML should be parsed via fallback."""
+        content = yaml.dump({"key": "value", "nested": {"inner": 42}})
+        with tempfile.NamedTemporaryFile(suffix=".cfg", mode="w", delete=False) as f:
+            f.write(content)
+            f.flush()
+            result = load_file(f.name)
+        os.unlink(f.name)
+        assert result["key"] == "value"
+        assert result["nested.inner"] == 42
+
+    def test_fallback_yaml_as_json(self):
+        """Files with unknown ext containing valid JSON should be parsed via fallback."""
+        content = json.dumps({"host": "localhost", "port": 8080})
+        with tempfile.NamedTemporaryFile(suffix=".cnf", mode="w", delete=False) as f:
+            f.write(content)
+            f.flush()
+            result = load_file(f.name)
+        os.unlink(f.name)
+        assert result["host"] == "localhost"
+        assert result["port"] == 8080
+
+    def test_unsupported_format_fallback_to_dotenv(self):
+        """Files with unknown extension and no structured content fall through to .env parser and returns a dict."""
+        with tempfile.NamedTemporaryFile(suffix=".xyz", mode="w", delete=False) as f:
+            f.write("not a config")
+            f.flush()
+            result = load_file(f.name)
+        os.unlink(f.name)
+        assert isinstance(result, dict)
+
+    def test_unsupported_format_empty_result(self):
+        """Garbage content with unknown extension returns empty dict via .env fallback."""
+        with tempfile.NamedTemporaryFile(suffix=".xyz", mode="w", delete=False) as f:
+            f.write("!!!garbage!!!")
+            f.flush()
+            result = load_file(f.name)
+        os.unlink(f.name)
+        assert result == {}
+
+    def test_yaml_non_dict_raises(self):
+        """YAML containing a list (not a mapping) should raise ValueError."""
+        content = yaml.dump(["item1", "item2"])
+        with tempfile.NamedTemporaryFile(suffix=".yaml", mode="w", delete=False) as f:
+            f.write(content)
+            f.flush()
+            with pytest.raises(ValueError, match="YAML file must contain a mapping"):
+                load_file(f.name)
+        os.unlink(f.name)
+
+
 class TestFlattenNested:
     def test_flatten(self):
         data = {"a": {"b": {"c": 1}, "d": 2}, "e": 3}