Some cleanup

rest_framework/fields.py

@@ -181,7 +181,7 @@ class WritableField(Field):
 
         try:
             if self._use_files:
-                files = files or {}  # For the case when files==None
+                files = files or {}
                 native = files[field_name]
             else:
                 native = data[field_name]

rest_framework/tests/files.py

@@ -46,6 +46,6 @@ class FileSerializerTests(TestCase):
         """
         now = datetime.datetime.now()
 
-        serializer = UploadedFileSerializer(data={'created': now}, files=None)
+        serializer = UploadedFileSerializer(data={'created': now})
         self.assertFalse(serializer.is_valid())
         self.assertIn('file', serializer.errors)