I see the VIPS engine doing that for PNG requests:
wget http://127.0.0.1:8800/wikipedia/hu/thumb/4/4e/Otrs-admin.png/180px-Otrs-admin.png Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: thumbor:ERROR ERROR: Traceback (most recent call last): Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: File "/usr/lib/python2.7/dist-packages/thumbor/handlers/__init__.py", line 111, in get_image Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: self.context.request.image_url Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: File "/usr/lib/python2.7/dist-packages/tornado/gen.py", line 870, in run Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: value = future.result() Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: File "/usr/lib/python2.7/dist-packages/tornado/concurrent.py", line 215, in result Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: raise_exc_info(self._exc_info) Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: File "/usr/lib/python2.7/dist-packages/tornado/gen.py", line 879, in run Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: yielded = self.gen.send(value) Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: File "/usr/lib/python2.7/dist-packages/thumbor/handlers/__init__.py", line 525, in _fetch Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: self.context.request.engine.load(fetch_result.buffer, extension) Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: File "/usr/lib/python2.7/dist-packages/wikimedia_thumbor/engine/proxy/proxy.py", line 102, in load Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: enginename = self.select_engine() Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: File "/usr/lib/python2.7/dist-packages/wikimedia_thumbor/engine/proxy/proxy.py", line 63, in select_engine Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: if engine.should_run(self.lcl['buffer']): Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: File "/usr/lib/python2.7/dist-packages/wikimedia_thumbor/engine/vips/vips.py", line 80, in should_run Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: self.context.vips['width'] = int(size[0]) Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: ValueError: invalid literal for int() with base 10: '' Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: thumbor:ERROR [BaseHandler] get_image failed for url `http%3A//ms-fe.svc.eqiad.wmnet/v1/AUTH_mw/wikipedia-hu-local-public.4e/4/4e/Otrs-admin.png`. error: `invalid literal for int() with base 10: ''` Sep 8 11:18:28 thumbor1001 thumbor@8816[20745]: tornado.access:ERROR 500 GET /wikipedia/hu/thumb/4/4e/Otrs-admin.png/180px-Otrs-admin.png (127.0.0.1) 3198.22ms
At first glance it seems like the VIPS engine can't get the dimensions of the image to decide whether it'll run on it or not.