You're a lifesaver Cátia.
For anyone using get_posts or similar to manually loop through attachments, her solution still works. For example:
<?php
$thumb_id = get_post_thumbnail_id(get_the_ID()); // gets the post thumbnail ID
$args = array(
'order' => 'ASC',
'orderby' => 'menu_order',
'post_type' => 'attachment',
'post_parent' => $post->ID,
'post_mime_type' => 'image',
'post_status' => null,
'numberposts' => -1,
'exclude' => $thumb_id
);
$attachments = get_posts($args);
if ($attachments) {
foreach ($attachments as $attachment) {
echo wp_get_attachment_image($attachment->ID, 'medium', false);
}
}
?>