Index: post.php
===================================================================
--- post.php	(revision 23460)
+++ post.php	(working copy)
@@ -4350,6 +4350,8 @@
 
 	if ( !empty($imagedata['thumb']) && ($thumbfile = str_replace(basename($file), $imagedata['thumb'], $file)) && file_exists($thumbfile) )
 		return apply_filters( 'wp_get_attachment_thumb_file', $thumbfile, $post->ID );
+	elseif ( isset($imagedata['sizes']['thumbnail']['file']) && !empty($imagedata['sizes']['thumbnail']['file']) && ($thumbfile = str_replace(basename($file), $imagedata['sizes']['thumbnail']['file'], $file)) && file_exists($thumbfile) )
+		return apply_filters( 'wp_get_attachment_thumb_file', $thumbfile, $post->ID );
 	return false;
 }