function sd_edd_download_details_widget_thumbnail( $instance ) {

	$download_id = isset( $instance['download_id'] ) ? $instance['download_id'] : 0;

	if ( 'current' === $instance['display_type'] ) {

		$download_id = get_the_ID();

	}

	if ( empty( $download_id ) || ! has_post_thumbnail( $download_id ) || 'download' !== get_post_type( $download_id ) ) {

		return;

	}

	echo get_the_post_thumbnail( $download_id );

}

add_action( 'edd_product_details_widget_before_purchase_button', 'sd_edd_download_details_widget_thumbnail' );