How to change value of the field in a proxy model in Django
===

I have a base model, and proxy models. I want to change value of upload_to of 'image' filed, that depends on 'file_type' filed.
I'm not sure how to do this, do I need to specify it in my .Manager class or in Proxy class. 
Can you please give me same detail how to do this.

    class FileDescription(models.Model):
        
        class Meta:
            ordering = ['file_creation_time']
            
        TYPE_CHOICES = (
            ('homework', 'Homework'),
            ('class', 'Class Papers'),
            ('random', 'Random Papers')                    
        )
            
        subject = models.ForeignKey('Subjects', null=True, blank=True, primary_key=True)
        subject_name = models.CharField(max_length=100, unique=False)
        file_type = models.CharField(max_length=100, choices=TYPE_CHOICES) 
        ....
        ....
        image = models.ImageField(upload_to= ???????????      blank=True, null=True, max_length=255)
        
        def __unicode__(self):
            return u'%s' % (self.file_name)
        
    class HomeworkManager(models.Manager):
        def get_query_set(self):
            qs = super(HomeworkManager, self).get_query_set().filter(file_type='homework')
            return qs
        
    class ClassPapersManager(models.Manager):
        def get_query_set(self):
            qs = super(HomeworkManager, self).get_query_set().filter(file_type='class')
            return qs
        
    class RandomPapersManager(models.Manager):
        def get_query_set(self):
            qs = super(HomeworkManager, self).get_query_set().filter(file_type='random')
            return qs
        
    class Homework(FileDescription):
    
        class Meta:
            proxy = True