Index: post.php
===================================================================
--- post.php	(revision 18191)
+++ post.php	(working copy)
@@ -3882,7 +3882,7 @@
 
 	$file = get_attached_file( $post->ID );
 
-	if ( !empty($imagedata['thumb']) && ($thumbfile = str_replace(basename($file), $imagedata['thumb'], $file)) && file_exists($thumbfile) )
+	if ( !empty($imagedata['thumbnail']) && ($thumbfile = str_replace(basename($file), $imagedata['thumbnail'], $file)) && file_exists($thumbfile) )
 		return apply_filters( 'wp_get_attachment_thumb_file', $thumbfile, $post->ID );
 	return false;
 }