attachment_fu, temporary uploads, and RMagick manipulations

I want to provide users the option to upload images and have them manipulated with RMagick before scaling them down. The problem is, I'm not sure how to set up the controller to treat images that aren't passed through the manipulation stage.

Users: 1. upload an image with a form, through which the image is resized to something manageable with attachment_fu's :resize_to, 2. are thereafter redirected_to to a new action where they can choose to manipulate the picture with RMagick filter(s), 3. after choosing to manipulate the image (or not), the image is scaled down further.

Basically, I don't want pictures to stay big for long, but if the user navigates away from the manipulation action, the image is never scaled- down.

How do I ensure that images are scaled down a second time? Is this a job for an app-wide before_filter?