Django create thumbnail form ImageField and save in a different ImageField - now with updating!

imagewiththumbnails_updateable.py

def create_thumbnail(instance, image_field, thumb_field):
    # original code for this method came from
    # http://snipt.net/danfreak/generate-thumbnails-in-django-with-pil/
    # https://gist.github.com/valberg/2429288
    
    # adapted to python 3
    
    # insert this code to utils.py
    
    # example:
    # instance = Photo.objects.get(id=1)
    # create_thumbnail(instance, 'photo', 'photo_thumbnail')
    
    # example 2:
    # signals.py:
    # from fieldsignals import post_save_changed   # https://github.com/craigds/django-fieldsignals
    # @receiver(post_save_changed, sender=SeminarPhotos)
    # def set_thumbnail(sender, instance, changed_fields=None, **kwargs):
    #     names = [i.name for i in changed_fields.keys()]
    #     if 'photo' in names and instance.photo:
    #         create_thumbnail(instance, 'photo', 'photo_thumbnail')

    
    # If there is no image associated with this.
    # do not create thumbnail
    if not getattr(instance, image_field, None):
        return None

    # Set our max thumbnail size in a tuple (max width, max height)
    thumbnail_size = (150, 150)

    mime = Magic(mime=True)
    django_type = mime.from_file(getattr(instance, image_field).file.name)

    if django_type == 'image/jpeg':
        pil_type = 'jpeg'
    elif django_type == 'image/png':
        pil_type = 'png'
    elif django_type == 'image/x-ms-bmp':
        pil_type = 'bmp'
    elif django_type == 'image/gif':
        pil_type = 'gif'
    elif django_type == 'image/tiff':
        pil_type = 'tiff'
    else:
        return None

    image = Image.open(getattr(instance, image_field).file.name)
    thumb = ImageOps.fit(image, thumbnail_size, Image.ANTIALIAS)

    buffer = BytesIO()
    thumb.save(buffer, pil_type)
    buffer.seek(0)

    suf = SimpleUploadedFile(os.path.split(getattr(instance, image_field).name)[-1],
                             buffer.read(), content_type=django_type)

    setattr(instance, thumb_field, suf)
    instance.save()