Validation of files uploaded via an admin form

Hi, I'm having a little difficulty with validating a file
which is 
uploaded by the user in the admin centre. Basically, I've
created a 
small model which stores data about a music file upload. The

documentation for FileField (which I'm using) says (I
quote):
'Validate all uploaded files so that you're sure the files
are what 
you think they are...'

However, I can't get this to work. I have created a custom
validator, 
which checks whether the filetype is of mp3 or ogg, and that
the 
extension is 'mp3' or 'ogg'. This works fine for the initial
upload 
(and indeed I can upload files!), but when I go to the
'edit' part of 
the admin and upload a new file, I get a TypeError. This is
because in 
my validator, I check the field_data the first time for
'filename' and 
'content-type', because field_type is a dictionary, but it
seems that 
on an edit, field_type is a string with the filename of the
_previous_ 
file which was uploaded. Why?!

Anyway, my question is: how can I complete this (seemingly)
relatively 
simple task of validation?

Here's my code so far: http://dpaste.com/5068/

Any help would be much appreciated.

--
Isaac


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