(Bug 5135) Add bulk resized image support

This lets the /file/edit page load resized images so it doesn't force
you to load the full image. Additionally, this makes it so that the URL
returned is always the same as the input width/height, which makes for a
more consistent UI.

This also fixes the image/x-png MIME type that File::Type returns, which
causes Chrome to want to download the image.