title: attachment_fu: Don't reload thumbnails
body: <p>I've got a nice little "photos" class that has attached images. When I go to the page to sort the photos' order, it iterates though each photo, sets the new "sort" value, and saves it. All good so far.</p>
  
  <p>Problem is, I've noticed this behavior is fairly slow. Turns out, attachment_fu reloads the thumbnail on every save - regardless of whether or not there's new image data to work with.</p>
  
  <p>Obviously this system has been well-thought-out, so I'm only left to assume that a provision exists for this situation. How do I tell attachment_fu not to regenerate the thumbnails when it's not appropriate?</p>
  
  <p>Thanks, <em>--Matchu</em></p>
  
  <p><strong>Edit:</strong> I just remembered that, for this particular situation, I can use update_attribute to dodge all the validations and other callbacks. However, this isn't really a viable answer to the whole big scenario. What am I missing?</p>