Source

File: wp-includes/class-wp-image-editor-gd.php

public function save( $filename = null, $mime_type = null ) {
		$saved = $this->_save( $this->image, $filename, $mime_type );

		if ( ! is_wp_error( $saved ) ) {
			$this->file      = $saved['path'];
			$this->mime_type = $saved['mime-type'];
		}

		return $saved;
	}