asset cache (timestamp after the image link)

For browser cache purpose, asset_helper help us to add a timestamp
(integer) after image, javascript and css files
(, but which seems doesn't
working properly in my production server.

I am not sure if I am correct, it check the file last modify time to
generate the integer. Our problem is, we use attachment_fu to upload
photo, and we added a feature to rotate the photo. I checked the photo
in file system(linux), and the database filed (updated_at), both updated
after the photo is rotated, but the timestamp in the url of the image
does not update......

We use mongrel cluster and deploy using capistrano. We haven't override
any function about asset_helper.

Any idea?