title: ImageField: save filename only
body: <p>I have model with <code>ImageFile</code> field:</p>
  
  <pre><code>def upload_course_cover(object, filename):
      return '/media/courses/%s_%s' % (Course.objects.aggregate(Max('id'))['id__max'] + 1, filename)
  
  class Course(models.Model):
      # ...
      cover = models.ImageField(upload_to=upload_course_cover,  blank=True)
  </code></pre>
  
  <p>When the image is saved, into <code>cover</code> field will be writen full image path <code>/media/courses/id_filename.ext</code>, but I want store only image name <code>id_filename.ext</code>.
  How to do it?</p>