Wordpress enclosure workflow:


1) wp_insert_post (post.php)
       Sets the '_encloseme' meta_key if $post_status==publish and $post_type=post
2) do_all_pings action (comment.php)
		for each _encloseme flag
			a) Delete flag
			b) call do_enclose
3) do_enclose (functions.php)
     does some confusing parsing and testing (which looks like it looks at all http:// urls
     in the content whether they are enclosures or not)
     calls wp_get_http_headers on filtered $url values
     uses the results to set 'enclosure' meta_value with url, length, and content-type

4) wp_get_http_headers (functions.php)
     Reads headers from passed in URL using fsockopen() and fgets()
     returns an array of headers


1) wp-rss2.php and wp-atom.php
     call rss_enclosure()
2) rss_enclosure()
      call get_post_custom() to read $custom_fields 
      uses $custom_fields to build and display the enclosure tag