attachment_fields_to_edit not working with WP 3.5

Hi,

I had in my functions.php a small helper function working on the filter attachment_fields_to_edit which just fills some fields during media upload:

function add_image_attachment_fields_to_edit( $form_fields, $post ) {
    //$form_fields['post_content']['value'] = 'my-value';//Description
	$img_title = get_the_title($post->ID);
	$post_title = get_the_title($post->post_parent);
	$img_exif = get_wp_exif_tags($post->ID);
	if ($img_title <> ''){$post_title = $post_title .' - ' . $img_title;}
	if ($img_exif == ' , ') {$img_exif = '';} else {
			if ($post_title <> 'Auto Draft') {$img_exif = ' | ' . $img_exif;}
				else {$post_title = '';}
			}
    $form_fields['image_alt']['value'] = $post_title . $img_exif;
    //$form_fields['post_excerpt']['value'] = 'my-value'; //caption
    //$form_fields['url']['value'] = 'my-value';
    return $form_fields;
}
add_filter("attachment_fields_to_edit", "add_image_attachment_fields_to_edit", 15, 2);

But somehow this is not throwing any error message nor it is working – do you have any idea what might causing this not working?

Thanks, Sebastian