WordPress at a glance
function is not described

_wp_post_thumbnail_class_filter_add() WP 2.9.0

Adds '_wp_post_thumbnail_class_filter' callback to the 'wp_get_attachment_image_attributes' filter hook. Internal use only.

No Hooks.

Return

Nothing (null).

Usage

_wp_post_thumbnail_class_filter_add( $attr );
$attr(array) (required)
Thumbnail attributes including src, class, alt, title.

Code of wp post thumbnail class filter add: wp-includes/media.php VER 5.0.3

<?php
function _wp_post_thumbnail_class_filter_add( $attr ) {
	add_filter( 'wp_get_attachment_image_attributes', '_wp_post_thumbnail_class_filter' );
}