You’re missing posts
from the first argument in both the add_filter()
and add_action()
. It should follow the format of
So add_filter('manage_edit-document_columns', 'my_columns');
should be
add_filter('manage_edit-document_posts_columns', 'my_columns');
And add_action('manage_document_custom_column', 'my_custom_columns');
will read:
add_action('manage_document_posts_custom_column', 'my_custom_columns');