Configure addition reverse-proxy options with static-darkhttpd

[CrushedPixel]

I'm using static-darkhttpd to serve a static frontend like this:

  nixcloud.webservices.static-darkhttpd.frontend = {
    enable = true;
    root = /etc/nixos/modules/frontend-static;

    proxyOptions = {
      inherit domain;

      port = frontend_port;
      path = "/";
      TLS = domain;
      https.mode = "on";
    };
  };

This works very well.
Now I want to add a custom option to the reverse-proxy entry for this frontend, namely something like try_files $url $url.html to allow users to access the pages served without the .html extension.

(How) is this possible?