WordPress at a glance
function is not described

_wp_preview_post_thumbnail_filter() WP 4.6.0

Filters post thumbnail lookup to set the post thumbnail.

No Hooks.

Return

null/Array. The default return value or the post thumbnail meta array.

Usage

_wp_preview_post_thumbnail_filter( $value, $post_id, $meta_key );
$value(null/array/string) (required)
The value to return - a single metadata value, or an array of values.
$post_id(int) (required)
Post ID.
$meta_key(string) (required)
Meta key.

Code of wp preview post thumbnail filter: wp-includes/revision.php VER 5.1.1

<?php
function _wp_preview_post_thumbnail_filter( $value, $post_id, $meta_key ) {
	if ( ! $post = get_post() ) {
		return $value;
	}

	if ( empty( $_REQUEST['_thumbnail_id'] ) ||
		empty( $_REQUEST['preview_id'] ) ||
		$post->ID != $post_id ||
		'_thumbnail_id' != $meta_key ||
		'revision' == $post->post_type ||
		$post_id != $_REQUEST['preview_id'] ) {

		return $value;
	}

	$thumbnail_id = intval( $_REQUEST['_thumbnail_id'] );
	if ( $thumbnail_id <= 0 ) {
		return '';
	}

	return strval( $thumbnail_id );
}