In the admin, give media list table icons `auto` for `width` and `height`. In the post thumbnail metabox, apply `width: auto`. 

If someone turns on support SVG files, this will allow them to show up.

Props arippberger for an initial patch.
Fixes #26256.


git-svn-id: https://develop.svn.wordpress.org/trunk@29832 602fd350-edb4-49c9-b593-d223f7449a82