AppImage Wrapper: needed library not found despite being added

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Trying to build a wrapper package for an AppImage I’m running into a problem that at runtime apparently a lib can’t be found:

#configuration.nix
{ config, lib, pkgs, ... }:

{
  imports =
    [ # Include the results of the hardware scan.
      ./appimages.nix
    ];

#appimages.nix
{ pkgs, lib, inputs, ... }:

let 
	repetierApp = import ./appimages/repetier.nix { inherit pkgs; };
in
{

    programs.appimage.enable = true;
    programs.appimage.binfmt = true;

	environment.systemPackages = with pkgs; [
        libgdiplus
		repetierApp
	];
}

#appimages/repetier.nix
{ pkgs, ... }:
let
  pname = "repetier-host";
  version = "2.3.2";

  src = pkgs.fetchurl {
    url = "https://download3.repetier.com/files/host/linux/Repetier-Host-x86_64-${version}.AppImage";
    #nix-hash --type sha256 --flat --base64 Repetier-Host-x86_64-2.3.2.AppImage
    hash = "sha256-gvC432ioUA2lbbJKv6CJHD1OKMZZsqA7dUiVX4ygavc=";
  };
in
pkgs.appimageTools.wrapType2 {
  inherit pname version src;
  extraPkgs = pkgs: [
    pkgs.libgdiplus
  ];
}

When starting the app, it crashes with

 System.DllNotFoundException: libgdiplus.so.0 assembly:<unknown assembly> type:<unknown type> member:(null)

I’m not sure, if

programs.appimage.binfmt

and

  environment.systemPackages = with pkgs; [
        libgdiplus
  ];

is even needed when packaging AppImages this way, added it anyway.

Shouldn’t

  extraPkgs = pkgs: [
    pkgs.libgdiplus
  ];

Add the library to the final package env?