Category counter is not working for attachment post type

[JohnOak]

When registering a custom taxonomy for "attachment" post type:

register_taxonomy( 'test', 'attachment', [
	"label" => "Test",
	"singular_label" => "Test",
]);

And setting taxonomy value for attachment file:

The count for items in registered taxonomy shows 0 (if there is 1 or more items it will still show 0 an this is the problem):

When clicking on count (that shows 0) opens Media Library with items that have this taxonomy value. And in here everything is just fine:

The problem is that wrong counter that should be fixed.

Bug was tested with clean WordPress install with no plugins and on clean Twenty Fifteen child theme.

[swissspidy]

Please have a look at the documentation of the update_count_callback parameter in the WordPress Codex: ​https://codex.wordpress.org/Function_Reference/register_taxonomy. This is by design.

Because an attachment is a type of post, the default _update_post_term_count() will be used. However, this may be undesirable, because this will only count attachments that are actually attached to another post (like when you insert an image into a post). This means that attachments that you simply upload to WordPress using the Media Library, but do not actually attach to another post will not be counted.

[ocean90]

#40896 was marked as a duplicate.

[ocean90]

Related: #22558