Remove $post_type global usage from media list table. Correct $post_type to...

Remove $post_type global usage from media list table. Correct $post_type to 'attachment' for upload screens. see #18722.

git-svn-id: https://core.svn.wordpress.org/trunk@19385 1a063a9b-81f0-0310-95a4-ce76da25c4cd