WP_Image_Editor::generate_filename

	public function generate_filename( $suffix = null, $dest_path = null, $extension = null ) {
		// $suffix will be appended to the destination filename, just before the extension.
		if ( ! $suffix ) {
			$suffix = $this->get_suffix();
		}

		$dir = pathinfo( $this->file, PATHINFO_DIRNAME );
		$ext = pathinfo( $this->file, PATHINFO_EXTENSION );

		$name    = wp_basename( $this->file, ".$ext" );
		$new_ext = strtolower( $extension ? $extension : $ext );

		if ( ! is_null( $dest_path ) ) {
			$_dest_path = realpath( $dest_path );
			if ( $_dest_path ) {
				$dir = $_dest_path;
			}
		}

		return trailingslashit( $dir ) . "{$name}-{$suffix}.{$new_ext}";
	}