gistfile1.py

import os, uuid
from django.conf import settings
from django.db import models
from django.utils.encoding import smart_unicode, smart_str

class BannerFileField(models.FileField):
    def pre_save(self, model_instance, add):
        file = super(BannerFileField, self).pre_save(model_instance, add)
        
        if not add:
            old = model_instance.__class__.objects.get(pk=model_instance.pk)
            try:
                os.remove(getattr(old, self.name).path)
            except OSError:
                pass
        
        f_ext = file.name.split('.')[-1]

        while True:
            name = uuid.uuid1()
            f_name = name + u'.' + f_ext
            f_new = smart_unicode(os.path.join(settings.MEDIA_ROOT, self.upload_to, f_name)
            if not os.path.exists(f_new):
                break
        
        os.rename(file.path, f_new)
        upload_to = self.upload_to
        if not upload_to.endswith(u'/'): upload_to = upload_to + u'/'
        file.name = self.upload_to + u'/' + f_name
        return file