WordPress at a glance
function is not described

_wp_get_attachment_relative_path() WP 4.4.1

Get the attachment path relative to the upload directory.

No Hooks.

Return

String. Attachment path relative to the upload directory.

Usage

_wp_get_attachment_relative_path( $file );
$file(string) (required)
Attachment file name.

Code of wp get attachment relative path: wp-includes/media.php VER 5.1.1

<?php
function _wp_get_attachment_relative_path( $file ) {
	$dirname = dirname( $file );

	if ( '.' === $dirname ) {
		return '';
	}

	if ( false !== strpos( $dirname, 'wp-content/uploads' ) ) {
		// Get the directory name relative to the upload directory (back compat for pre-2.7 uploads)
		$dirname = substr( $dirname, strpos( $dirname, 'wp-content/uploads' ) + 18 );
		$dirname = ltrim( $dirname, '/' );
	}

	return $dirname;
}