Fixed #21042 -- Allowed accessing FileDescriptor on the model class.

This is consistent with ability to reference other descriptors
on the model class (5ef0c03ae9).

tests/file_storage/tests.py

@@ -18,6 +18,7 @@ from django.core.files.storage import FileSystemStorage, get_storage_class
 from django.core.files.uploadedfile import (
     InMemoryUploadedFile, SimpleUploadedFile, TemporaryUploadedFile,
 )
+from django.db.models.fields.files import FileDescriptor
 from django.test import (
     LiveServerTestCase, SimpleTestCase, TestCase, override_settings,
 )
@@ -447,9 +448,7 @@ class FileFieldStorageTests(TestCase):
             return 255  # Should be safe on most backends
 
     def test_files(self):
-        # Attempting to access a FileField from the class raises a descriptive
-        # error
-        self.assertRaises(AttributeError, lambda: Storage.normal)
+        self.assertIsInstance(Storage.normal, FileDescriptor)
 
         # An object without a file has limited functionality.
         obj1 = Storage()