When rendering a list of attachments in `WP_Media_List_Table` class, none image attachments, show a generic icon. However, attachment types like audio and video support adding a featured image. For attachments that have featured images, us this image instead of the icon. This featured image is a better preview than a generic icon.
Props spacedmonkey, samful, johnbillion, JavierCasares, seanchayes, antpb, cadic, JeffPaul.
Fixes#49852.
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git-svn-id: http://core.svn.wordpress.org/trunk@54493 1a063a9b-81f0-0310-95a4-ce76da25c4cd