Filenames rarely have negative numbers in them, and it'd usually be ambiguous whether they were negative or dash-separated positive.

I know you jest, but this just further demonstrates why Natural Sorting is complicated and might not be the best default choice.

my_photos_at_-3c

my_photos_at_-10c

Do users want smaller numbers first, or do they want them in counting order, away from zero?

That's a hyphen, not a minus sign, silly.