Got really excited with the new version of WP 3.5.
New media organizer (love it), better dashboard, retina display(Apple) ready page elements.
Well, it’s not missing, it’s just that some plugins messed up your folder access.
One of them is BulletProof Security. Another one is WordPress Gallery Plugin.
How do you fix that?
First, add this line to wp-config.php:
Then, deactivate BulletProof Security and WordPress Gallery Plugin.
Go back to updates and reinstall WP.
Then remove the line of code you added to wp-config.php.
Now you should be fine.