在保存对象之前处理文件上载

I've got a model like this:

我有一个这样的模型:

class Talk(BaseModel):
  title        = models.CharField(max_length=200)
  mp3          = models.FileField(upload_to = u'talks/', max_length=200)
  seconds      = models.IntegerField(blank = True, null = True)
clas