WP_Filesystem_FTPext::exists( string $file )

	public function exists($file) {
		$list = @ftp_nlist($this->link, $file);

		if ( empty( $list ) && $this->is_dir( $file ) ) {
			return true; // File is an empty directory.
		}

		return !empty($list); //empty list = no file, so invert.
	}