title: How can I alter a models.FileField in a pre_save signal?
body: <p>I have a model field defined like the following:</p>
  
  <pre><code>file = models.FileField(storage=S3BotoStorage(bucket=blablabla), upload_to='blablabla', max_length=250)
  </code></pre>
  
  <p>What I am trying to do is grabbing the <code>file</code> in a <code>pre_save</code> and compress it then save the zipped version.</p>
  
  <p>You may say I should do that in my views but that is not possible since I am using Django admin site and I need to have any file saved throughout the code whether from the views or Django admin site compressed.</p>
  
  <p>So, how can I access the uploaded file, compress it and have it saved instead of the original file?</p>