Don't set the poster image for videos in a playlist if the image is the video mime-type icon blown up.

Merges [28158] to the 3.9 branch.

props wonderboymusic.
see #27892.

Built from https://develop.svn.wordpress.org/branches/3.9@28246


git-svn-id: http://core.svn.wordpress.org/branches/3.9@28074 1a063a9b-81f0-0310-95a4-ce76da25c4cd