Apply a subtle semi-transparent border to images in the media list table to avoid the "white on white (like a polar bear in Antarctica)" problem.

Props ericlewis.
Fixes #15860.

Built from https://develop.svn.wordpress.org/trunk@28842


git-svn-id: http://core.svn.wordpress.org/trunk@28646 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Only apply a subtle semi-transparent border to attachments that are images in the media list table.

See [28842], #15860.


Built from https://develop.svn.wordpress.org/trunk@28843


git-svn-id: http://core.svn.wordpress.org/trunk@28647 1a063a9b-81f0-0310-95a4-ce76da25c4cd