title: Silverstripe File onAfterUpload method not called
body: <p>I have an extension to File added with</p>
  
  <pre><code>Object::add_extension('File', 'MyFileDecorator');
  </code></pre>
  
  <p>I can confirm that the extension class is being created and that extending methods <code>onBeforeWrite</code> is working fine.</p>
  
  <p>However, I am never seeing an extended <code>onAfterUpload</code> being called, despite the documentation claiming it should be called when the file has been uploaded.</p>
  
  <p>I have tried using <code>FileIFrameField</code>, <code>FileUploadField</code> (from uploadify) and various data object manager classes, all to no avail.</p>
  
  <p>Looking through <code>FileIFrameField</code>, it looks like there's no call to <code>onAfterUpload</code> so I guess my question is whether there's a field I could use that does call that method?</p>