I've got a model like this:

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)

I want to validate before saving that the uploaded file is an MP3, like this:

def is_mp3(path_to_file):
  from mutagen.mp3 import MP3
  audio = MP3(path_to_file)
  return not audio.info.sketchy

Once I'm sure I've got an MP3, I want to save the length of the talk in the seconds attribute, like this:

audio = MP3(path_to_file)
self.seconds = audio.info.length

The problem is, before saving, the uploaded file doesn't have a path (see this ticket, closed as wontfix), so I can't process the MP3.

I'd like to raise a nice validation error so that ModelForms can display a helpful error ("You idiot, you didn't upload an MP3" or something).

Any idea how I can go about accessing the file before it's saved?

^{p.s. If anyone knows a better way of validating files are MP3s I'm all ears - I also want to be able to mess around with ID3 data (set the artist, album, title and probably album art, so I need it to be processable by mutagen).}