'File storage can get a mixin to extend the functionality of the
returned file.'

def test_file_with_mixin(self):

    self.assertFalse(self.storage.exists('test.file'))
    class TestFileMixin(object, ):
        mixed_in = True
    f = ContentFile('custom    contents')
    f_name = self.storage.save('test.file', f)
    self.assert_(isinstance(self.storage.open('test.file', mixin=TestFileMixin), TestFileMixin))
    self.storage.delete('test.file')
