Fixed #11857 -- Added missing 'closed' property on TemporaryFile class.

- TemporaryFile now minimally mocks the API of the Python standard
  library class tempfile.NamedTemporaryFile to avoid AttributeError
  exceptions.
- The symbol django.core.files.NamedTemporaryFile is actually assigned
  as a different class on different operating systems.
- The bug only occurred if Django is running on Windows, hence why it
  was hard to diagnose.

django/core/files/temp.py

@@ -46,6 +46,15 @@ if os.name == 'nt':
                 except (OSError):
                     pass
 
+        @property
+        def closed(self):
+            """
+            This attribute needs to be accessible in certain situations,
+            because this class is supposed to mock the API of the class
+            tempfile.NamedTemporaryFile in the Python standard library.
+            """
+            return self.file.closed
+
         def __del__(self):
             self.close()