Why isn't Django's `ImageField` throwing a validation error here?

```py
    # field in model
    image_mobile = ImageField(
        upload_to='static/images/',
        blank=True,
        null=True
    )

    # in test
    from django.core.files.uploadedfile import SimpleUploadedFile

    attachment = SimpleUploadedFile("file.mp4", b"file_content", content_type="text/plain")
    obj.image_mobile = attachment
    obj.save()
    self.assertEqual(obj.image_mobile, '')

```

Outputs this:

```
AssertionError: <ImageFieldFile: static/images/file_7wanB5P.mp4> != ''

```

From the documentation:

> 
> Inherits all attributes and methods from FileField, but also validates that the uploaded object is a valid image.
> 
> 
>