Twenty Twelve: Use the correct body class for attachments.

.single-attachment is only used when the attachment's parent is not a page.

props Marco_Teethgrinder.
fixes #25506.

Built from https://develop.svn.wordpress.org/trunk@25707


git-svn-id: http://core.svn.wordpress.org/trunk@25621 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-content/themes/twentytwelve/style.css

@@ -1454,7 +1454,7 @@ img#wpstats {
 		width: 65.104166667%;
 	}
 	body.template-front-page .site-content,
-	body.single-attachment .site-content,
+	body.attachment .site-content,
 	body.full-width .site-content {
 		width: 100%;
 	}