feat: expose immutable postbox file references

tests/test_router_contract.py

@@ -55,7 +55,7 @@ class FilesRouterContractTests(unittest.TestCase):
         actual = self._operation_keys(router)
 
         self.assertEqual(expected, actual)
-        self.assertEqual(53, len(actual))
+        self.assertEqual(54, len(actual))
         self.assertFalse(
             [operation for operation, count in Counter(actual).items() if count > 1]
         )
@@ -107,6 +107,16 @@ class FilesRouterContractTests(unittest.TestCase):
         self.assertIn((("DELETE",), "/files/{file_id}/shares/{share_id}"), routes)
         self.assertIn((("GET",), "/files/{file_id}/share-target-options"), routes)
 
+    def test_exact_version_download_route_is_exposed(self) -> None:
+        routes = {
+            (tuple(sorted(route.methods or ())), route.path) for route in router.routes
+        }
+
+        self.assertIn(
+            (("GET",), "/files/{file_id}/versions/{version_id}/download"),
+            routes,
+        )
+
     def test_file_listing_exposes_structured_property_filters(self) -> None:
         route = next(
             route