testing for a blank file upload

Question

I have a model that looks like this:

class ProjectImage(models.Model):
    project = models.ForeignKey(Project)
    in_slideshow = models.BooleanField(default=False)
    large_thumb = ImageWithThumbnailsField(
        upload_to='projects', 
        thumbnail={'size': (500, 384), 'options': ['crop']},
        generate_on_save=True,
    )
    medium_thumb = ImageWithThumbnailsField(
        upload_to='projects', 
        thumbnail={'size': (107, 107), 'options': ['crop']},
        generate_on_save=True,
        blank=True,
    )
    small_thumb = ImageWithThumbnailsField(
        upload_to='projects', 
        thumbnail={'size': (68, 68), 'options': ['crop']},
        generate_on_save=True,
        blank=True,
    )

    def save(self, force_insert=False, force_update=False):
        if self.medium_thumb is None:
            self.medium_thumb = self.large_thumb

        if self.small_thumb is None:
            self.small_thumb = self.medium_thumb
        super(ProjectImage, self).save(force_insert, force_update)

However, when I save without putting in anything for medium or small thumb, the code (as I expect it) does not provide the medium / small thumbs with the same file as the large thumb.

Thanks in advance