"uploaded" files are not cleaned up after a long time and even a update.php

if i upload an attachment, but no save the post, then the uploaded file will stay in my uploads/ file directory. after about a month, when I look at the uploads/ directory, those files are still there even though i did not save my post when i was uploading them, why are they not cleaned up? i offlined my drupal and run cron and update.php, none helped. is manually-removing-those-files the only choice?
I'd expect that if i did not save the post, those files should be purged during next cron job, or at least when i upgrade my drupal to a newer version, but that did not happen.