Tweak failing test

I've worked out this test is failing when FileDeleteView::post() calls
self.get_object() - that's when the 404 is being sent.  I can't work out
why though, because everything else looks fine.

apps/files/tests.py

@@ -53,13 +53,15 @@ class FileTests(TestCase):
             })
 
             data = response.json()
-
+            self.assertIsInstance(data['id'], int)
             self.assertEqual(data['is_valid'], True)
 
         response = self.client.post(reverse('files:delete', kwargs={
             'pk': data['id']
         }))
 
+        self.assertEqual(response.status_code, 200)
+
         data = response.json()
 
         self.assertEqual(data['success'], True)