Re-add sort by name to ML non-simple filter.

When replacing 'sort by extension' with
'sort by name' in simple filter mode, 'sort by name'
was erroneously removed from non-simple filter
mode too (and replaced by a second incarnation of
'sort by extension').

Signed-off-by: Thomas Hochstein <thh@inter.net>

include/functions_images.inc.php

@@ -1988,7 +1988,7 @@ function serendipity_getImageFields() {
                                          'type' => 'date'
                                    ),
 
-            'i.extension'         => array('desc' => SORT_ORDER_EXTENSION
+            'i.name'              => array('desc' => SORT_ORDER_NAME
                                    ),
 
             'i.authorid'          => array('desc' => AUTHOR,