I'm not picky on the file format. Something humanly readable (JSON, XML, etc) would be nice. But since it's primarily to allow me to save a configuration and restore it later, it's most important that your WWW site understand the format for the restore operation.
Something that is humanly readable, though, would allow me to save the configuration in, say, a git repository, which would allow me to see what I changed over time. And that would be a nice extra. JSON is probably best, but I'm flexible.
Thanks so much for considering this feature!