Source

File: wp-includes/post.php

function wp_get_attachment_thumb_file( $post_id = 0 ) {
	$post_id = (int) $post_id;
	$post    = get_post( $post_id );
	if ( ! $post ) {
		return false;
	}

	$imagedata = wp_get_attachment_metadata( $post->ID );
	if ( ! is_array( $imagedata ) ) {
		return false;
	}

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

	if ( ! empty( $imagedata['thumb'] ) ) {
		$thumbfile = str_replace( wp_basename( $file ), $imagedata['thumb'], $file );
		if ( file_exists( $thumbfile ) ) {
			/**
			 * Filters the attachment thumbnail file path.
			 *
			 * @since 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;
}