title: ImageField overwrite image file with same name
body: <p>I have model <code>UserProfile</code> with field <code>avatar = models.ImageField(upload_to=upload_avatar)</code></p>
  
  <p><code>upload_avatar</code> function names image file according <code>user.id</code> (12.png for example). </p>
  
  <p>But when user updates the avatar, new avatar name coincide with old avatar name and Django adds suffix to file name (12-1.png for example). </p>
  
  <p>There are way to <strong>overwrite file instead of create new file</strong>?</p>