Thunar
Thunar is a GTK file manager originally made for Xfce.
Installation
Thunar will automatically be installed by enabling Xfce.
If you want to install Thunar standalone, add to your configuration.nix:
programs.thunar.enable = true;
Plugins
You can add plugins by including them in programs.thunar.plugins to your configuration.nix. For example:
programs.thunar.plugins = with pkgs.xfce; [ thunar-archive-plugin thunar-volman ];
Other functionalities
You can extend Thunar's functionalities by adding to your configuration.nix:
services.gvfs.enable = true; # Mount, trash, and other functionalities
services.tumbler.enable = true; # Thumbnail support for images
You can extend Tumbler's support to other file formats by adding more packages to environment.systemPackages. See here for a list (the names may not match 1:1).