Add test for mtime change invalidating token

titofisto/tests.py

 from datetime import datetime, timedelta
 from io import BytesIO
+from pathlib import Path
 from unittest import TestCase
 
 from freezegun import freeze_time
@@ -125,3 +126,17 @@ class TitofistoTestCase(TestCase):
             res = self.client.get(url)
 
         self.assertEqual(res.status_code, 404)
+
+    def test_get_after_mtime_change(self):
+        """A file can not be retrieved with the same token after its mtime changes"""
+        now = datetime.now()
+        token, ts = self.storage.get_token(self.test_file_1_name, now.strftime("%s"))
+
+        url = (f"{settings.MEDIA_URL}{self.test_file_1_name}?"
+               f"{self.param_prefix}token={token}&"
+               f"{self.param_prefix}ts={ts}")
+        with freeze_time(now + timedelta(seconds=10)):
+            Path(self.storage.path(self.test_file_1_name)).touch()
+        res = self.client.get(url)
+
+        self.assertEqual(res.status_code, 404)