Index: src/wp-admin/includes/ajax-actions.php
===================================================================
--- src/wp-admin/includes/ajax-actions.php	(revision 35702)
+++ src/wp-admin/includes/ajax-actions.php	(working copy)
@@ -2570,7 +2570,7 @@
 	$rel = '';
 	$url = empty( $attachment['url'] ) ? '' : $attachment['url'];
 	if ( strpos( $url, 'attachment_id') || get_attachment_link( $id ) == $url ) {
-		$rel = ' rel="attachment wp-att-' . $id . '"';
+		$rel = 'attachment wp-att-' . $id;
 	}
 
 	remove_filter( 'media_send_to_editor', 'image_media_send_to_editor' );