test_something.py

class MyModelTest(SimpleTestCase):
    def test_something_with_some_file(self):
        my_model = MyModel()
        my_model.save()
        
        # We change the storage location to save into our test directory.
        my_model._meta.get_field('some_file').storage.location = '/somewhere/else'
        
        a_file = '/whatever/is/tosave.txt'
        my_model.some_file.save(
            os.path.basename(a_file),
            File(open(a_file, "r"))
        )
        
        self.assertTrue(os.path.exists('/somewhere/else/downloads/tosave.txt'))