I used to use attachment_fields_to_edit filter, but it seems that in 3.5+ it has been dropped. What is the new way of adding additional attachment fields?
It seems that it wasn't dropped, but - deprecated. In 3.6 it operates in compatibility mode. To make a new field appear in media popup, one should set additional param -
show_in_modal to true (helpful post here):
$form_fields['thumb-url'] = array( 'label' => __('Thumbnail Url'), 'input' => 'html', 'html' => <p> ... some html here ... </p>, 'show_in_modal' => true // <- this one );
But - still, the whole thing is deprecated and doomed to be dropped sooner or later. There should be a new way of doing things. Anyone knows what that'd be?