How does GTK+3 want to open your file?

main.c

/*
 * gcc -g `pkg-config gtk+-3.0 --libs --cflags` main.c -o main
 */

#include <err.h>
#include <stddef.h>
#include <stdlib.h>

#include <gtk/gtk.h>

extern int errno;

__dead static void	usage();

int
main(int argc, char *argv[])
{
	GAppInfo	*app_info;
	GFile		*file;
	GError		*error;

	if (argc < 2)
		usage();

	file = g_file_new_for_uri(argv[1]);

	error = NULL;
	app_info = g_file_query_default_handler(file, NULL, &error);
	g_object_unref(file);

	if (app_info == NULL)
		errx(1, "could not get app info from URI scheme: %s",
		    error->message);

	printf("exec: '%s'\n", g_app_info_get_commandline(app_info));

	g_object_unref(app_info);

	return 0;
}

static void
usage()
{
	fprintf(stderr, "usage: ./main file-uri\n");
	exit(64);
}