Update about page for new screenshot and new video encoding. Remove old screenshots directory. fixes #24832.

props lancewillett, markjaquith.

Merges [24942] to the 3.6 branch.



git-svn-id: https://core.svn.wordpress.org/branches/3.6@24943 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Use wp_get_mime_types() when calling wp_check_filetype() in the audio and video shortcodes. fixes #24911.

Merges [24911] to the 3.6 branch.



git-svn-id: https://core.svn.wordpress.org/branches/3.6@24945 1a063a9b-81f0-0310-95a4-ce76da25c4cd