WordPress at a glance
function is not described

_wp_post_thumbnail_class_filter() WP 2.9.0

Adds a 'wp-post-image' class to post thumbnails. Internal use only.

Uses the 'begin_fetch_post_thumbnail_html' and 'end_fetch_post_thumbnail_html' action hooks to dynamically add/remove itself so as to only filter post thumbnails.

This is an internal function for using it by WP core itself. It's not recommended to use this function in your code.

No Hooks.

Return

String[]. Modified array of attributes including the new 'wp-post-image' class.

Usage

_wp_post_thumbnail_class_filter( $attr );
$attr(string[]) (required)
Array of thumbnail attributes including src, class, alt, title, keyed by attribute name.

Changelog

Since 2.9.0 Introduced.

Code of _wp_post_thumbnail_class_filter() WP 5.7

<?php
function _wp_post_thumbnail_class_filter( $attr ) {
	$attr['class'] .= ' wp-post-image';
	return $attr;
}