feat(cli): automatically detect conversion format from DEST (#325)

* feat(cli): automatically detect conversion format from DEST

As titled

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fix(cli): fix and add tests

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* fix(ci): ?

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/test_main.py

@@ -64,6 +64,28 @@ def test_convert(slides_folder: Path, extension: str) -> None:
         assert results.exit_code == 0
 
 
+@pytest.mark.parametrize(
+    ("extension", "expected_log"),
+    [("html", ""), ("pdf", ""), ("pptx", ""), ("ppt", "WARNING")],
+)
+def test_convert_auto(slides_folder: Path, extension: str, expected_log: str) -> None:
+    runner = CliRunner()
+
+    with runner.isolated_filesystem():
+        results = runner.invoke(
+            cli,
+            [
+                "convert",
+                "BasicSlide",
+                f"basic_example.{extension}",
+                "--folder",
+                str(slides_folder),
+            ],
+        )
+
+        assert results.exit_code == 0, expected_log in results.output
+
+
 def test_init() -> None:
     runner = CliRunner()