Remove canonical redirect for uploads

If you upload an attachment and then delete it from disk, WordPress may think that you're actually referring to the attachment post itself. So we need to disable canonical redirects for URLs that are pointing inside of the uploads directory.

![local_wordpress_trunk_dev___just_another_wordpress_site](https://f.cloud.github.com/assets/134745/935851/650053a2-0092-11e3-8672-8ed276f15448.png)