Ah, apologies. I misread because I use nix-doom-emacs-unstraightened and it uses file path.
I checked the hm option and it is indeed just a long string. According to this comment, what you might be looking for is builtins.readFile. I tested on repl as below:
~ ❱ echo -en 'foo\nbar' > foobar
~ ❱ cat foobar
foo
bar⏎
~ ❱ nix repl
Lix 2.91.0
Type :? for help.
nix-repl> builtins.readFile ./foobar
"foo\nbar"
nix-repl>