I noticed that if I check the «Use content filter on full notes» option all image attachments showed up on site notes....
as mentioned in:
// TODO: content filter on attachment pages prepends a thumbnail for some reason.
// too much effort to find out why right now, so just don't apply content filter on attachment pages
After some tenacious Googlin...found this fix:
just add this:
$c = preg_replace('/<img[^>]+./','',$c);
$c = apply_filters('the_content', $c);
$c = str_replace(']]>', ']]>', $c);