If the ‘postcustom’ meta box is enabled, then we need to perform some extra initialization on it.

Filters the admin post thumbnail HTML markup to return.

Filters the size used to display the post thumbnail image in the ‘Featured image’ meta box.

Adds hidden input fields to the meta box save form.

Filters the default post content initially used in the “Write Post” form.

Filters the default post excerpt initially used in the “Write Post” form.

Filters the default post title initially used in the “Write Post” form.

Filters the number of items per page to show for a specific ‘per_page’ type.

Filters the number of posts displayed per page when specifically listing “posts”.

Fires right before the meta boxes are rendered.