Add a comment about `tmpname + '^'`.

tests/test_downloader_handlers.py

@@ -109,6 +109,7 @@ class FileTestCase(unittest.TestCase):
 
     def setUp(self):
         self.tmpname = self.mktemp()
+        # add a special char to check that they are handled correctly
         with open(self.tmpname + '^', 'w') as f:
             f.write('0123456789')
         handler = create_instance(FileDownloadHandler, None, get_crawler())