WordPress at a glance
filter is not described

attachment_url_to_postid filter-hook . WP 4.2.0

Filters an attachment id found by URL.

Usage

add_filter( 'attachment_url_to_postid', 'filter_function_name_8766', 10, 2 );
function filter_function_name_8766( $post_id, $url ){
	// filter...

	return $post_id;
}
$post_id(int/null)
The post_id (if any) found by the function.
$url(string)
The URL being looked up.

Where the hook is called

attachment_url_to_postid()
wp-includes/media.php 3943
return (int) apply_filters( 'attachment_url_to_postid', $post_id, $url );

Where the hook is used (in WP core)

Does not used.