Need to clear out inbox with procmail

I have a script that will extract attachments from emails stored in an an
mbox file. (/var/mail/username).
The following recipe is what I use to run that script whenever an email is
sent to the "upload" address:

:0
* ^TO(_dot_)*upload(_at_)example(_dot_)com
/var/mail/upload
{
  :0
  | /usr/local/bin/upload.sh
}

The problem is that I cannot seem to clear the contents of that mbox file
with Procmail or from any scripts. It's as if the file is protected until
Procmail is all finished. I need the file to be cleared out so the
attachments don't build-up in the mbox file. Any suggestions?

Nathan

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