Add a test for upgrading a room with a custom room type

tests/rest/client/test_upgrade_room.py

@@ -263,3 +263,33 @@ def test_space(self) -> None:
         self.assertIn((EventTypes.SpaceChild, self.room_id), state_ids)
         # The child that was removed should not be copied over.
         self.assertNotIn((EventTypes.SpaceChild, old_room_id), state_ids)
+
+    def test_custom_room_type(self) -> None:
+        """Test upgrading a room that has a custom room type set."""
+        test_room_type = "com.example.my_custom_room_type"
+
+        # Create a room with a custom room type.
+        room_id = self.helper.create_room_as(
+            self.creator,
+            tok=self.creator_token,
+            extra_content={
+                "creation_content": {EventContentFields.ROOM_TYPE: test_room_type}
+            },
+        )
+
+        # Upgrade the room!
+        channel = self._upgrade_room(room_id=room_id)
+        self.assertEqual(200, channel.code, channel.result)
+        self.assertIn("replacement_room", channel.json_body)
+
+        new_room_id = channel.json_body["replacement_room"]
+
+        state_ids = self.get_success(self.store.get_current_state_ids(new_room_id))
+
+        # Ensure the new room is the same type as the old room.
+        create_event = self.get_success(
+            self.store.get_event(state_ids[(EventTypes.Create, "")])
+        )
+        self.assertEqual(
+            create_event.content.get(EventContentFields.ROOM_TYPE), test_room_type
+        )