'InMemoryUploadedFile' object has no attribute 'storage'

I found error:

- You have an admin for a model
- In this admin, you have inline
- In this inline there is a ``ImageClearableFileInput`` widget

So, when I save invalid form with image to upload, it ends up in:

    thumb = self.get_thumbnail(value)
    thumbnailer.source_storage = value.storage

And since value is ``InMemoryUploadedFile``, it does not have storage attribute.

I think it may also happends with simplier form, while saving invalid form with image to upload.

I solved it just like Django does (https://github.com/django/django/blob/master/django/forms/widgets.py#L367) with check for ``url`` attribute.