wp_get_attachment_thumb_file( int $post_id )

function wp_get_attachment_thumb_file( $post_id = 0 ) {
	$post_id = (int) $post_id;
	if ( !$post = get_post( $post_id ) )
		return false;
	if ( !is_array( $imagedata = wp_get_attachment_metadata( $post->ID ) ) )
		return false;

	$file = get_attached_file( $post->ID );

	if ( !empty($imagedata['thumb']) && ($thumbfile = str_replace(basename($file), $imagedata['thumb'], $file)) && file_exists($thumbfile) ) {
		/**
		 * Filters the attachment thumbnail file path.
		 *
		 * @since WP-2.1.0
		 *
		 * @param string $thumbfile File path to the attachment thumbnail.
		 * @param int    $post_id   Attachment ID.
		 */
		return apply_filters( 'wp_get_attachment_thumb_file', $thumbfile, $post->ID );
	}
	return false;
}