Improved validation for received requests (#9817)

* Simplify `start_listening` callpath

* Correctly check the size of uploaded files

tests/unittest.py

@@ -247,6 +247,7 @@ class HomeserverTestCase(TestCase):
             config=self.hs.config.server.listeners[0],
             resource=self.resource,
             server_version_string="1",
+            max_request_body_size=1234,
             reactor=self.reactor,
         )