WordPress/wp-admin/js/set-post-thumbnail.js

function WPSetAsThumbnail(id){jQuery("#media-item-"+id+" > a.wp-post-thumbnail").text(setPostThumbnailL10n.saving).attr("href","#");jQuery.post(ajaxurl,{action:"set-post-thumbnail",post_id:post_id,thumbnail_id:id,cookie:encodeURIComponent(document.cookie)},function(str){var win=window.dialogArguments||opener||parent||top;if(str=="0"){alert(setPostThumbnailL10n.error);win.cwsCloseTB()}else{jQuery("a.wp-post-thumbnail").text(setPostThumbnailL10n.setThumbnail);jQuery("#media-item-"+id+" > a.wp-post-thumbnail").hide();win.WPSetThumbnailID(id);win.WPSetThumbnailHTML(str);win.WPCloseTB()}})};