better replacement of original form KeyError

src/flask/debughelpers.py

@@ -71,21 +71,25 @@ class FormDataRoutingRedirect(AssertionError):
 
 
 def attach_enctype_error_multidict(request):
-    """Since Flask 0.8 we're monkeypatching the files object in case a
-    request is detected that does not use multipart form data but the files
-    object is accessed.
+    """Patch ``request.files.__getitem__`` to raise a descriptive error
+    about ``enctype=multipart/form-data``.
+
+    :param request: The request to patch.
+    :meta private:
     """
     oldcls = request.files.__class__
 
     class newcls(oldcls):
         def __getitem__(self, key):
             try:
-                return oldcls.__getitem__(self, key)
+                return super().__getitem__(key)
             except KeyError as e:
                 if key not in request.form:
                     raise
 
-                raise DebugFilesKeyError(request, key) from e
+                raise DebugFilesKeyError(request, key).with_traceback(
+                    e.__traceback__
+                ) from None
 
     newcls.__name__ = oldcls.__name__
     newcls.__module__ = oldcls.__module__

src/flask/wrappers.py

@@ -110,7 +110,7 @@ class Request(RequestBase):
         return _split_blueprint_path(name)
 
     def _load_form_data(self) -> None:
-        RequestBase._load_form_data(self)
+        super()._load_form_data()
 
         # In debug mode we're replacing the files multidict with an ad-hoc
         # subclass that raises a different error for key errors.