diff --git a/nixos/modules/config/system-path.nix b/nixos/modules/config/system-path.nix
index a07a40e86bd1..6d17ca316e00 100644
--- a/nixos/modules/config/system-path.nix
+++ b/nixos/modules/config/system-path.nix
@@ -81,6 +81,12 @@ in
         description = "List of additional package outputs to be symlinked into <filename>/run/current-system/sw</filename>.";
       };
 
+      extraSetup = mkOption {
+        type = types.lines;
+        default = [ ];
+        description = "Shell fragments to be run after the system environment has been created. This should only be used for things that need to modify the internals of the environment, e.g. generating MIME caches. The environment being built can be accessed at $out.";
+      };
+
     };
 
     system = {
@@ -134,17 +140,8 @@ in
           if [ -x $out/bin/glib-compile-schemas -a -w $out/share/glib-2.0/schemas ]; then
               $out/bin/glib-compile-schemas $out/share/glib-2.0/schemas
           fi
-
-          if [ -x $out/bin/install-info -a -w $out/share/info ]; then
-            shopt -s nullglob
-            for i in $out/share/info/*.info $out/share/info/*.info.gz; do
-                $out/bin/install-info $i $out/share/info/dir
-            done
-          fi
-        '' ++ optionalString config.xdg.mime.enable ''
-          XDG_DATA_DIRS=$out/share ${pkgs.shared-mime-info}/bin/update-mime-database -V $out/share/mime > /dev/null
-
-          ${pkgs.desktop-file-utils}/bin/update-desktop-database $out/share/applications
+          
+          ${config.environment.extraSetup}
         '';
     };
 
diff --git a/nixos/modules/config/xdg/mime.nix b/nixos/modules/config/xdg/mime.nix
index db3e7c6d4c53..f1b672234a34 100644
--- a/nixos/modules/config/xdg/mime.nix
+++ b/nixos/modules/config/xdg/mime.nix
@@ -21,6 +21,16 @@ with lib;
       # this package also installs some useful data, as well as its utilities 
       pkgs.shared-mime-info 
     ];
+
+    environment.extraSetup = ''
+      if [ -w $out/share/mime ]; then
+        XDG_DATA_DIRS=$out/share ${pkgs.shared-mime-info}/bin/update-mime-database -V $out/share/mime > /dev/null
+      fi
+
+      if [ -w $out/share/applications ]; then
+        ${pkgs.desktop-file-utils}/bin/update-desktop-database $out/share/applications
+      fi
+    '';
   };
 
 }
diff --git a/nixos/modules/misc/documentation.nix b/nixos/modules/misc/documentation.nix
index b482a5a67523..e6ccda5d7f40 100644
--- a/nixos/modules/misc/documentation.nix
+++ b/nixos/modules/misc/documentation.nix
@@ -82,6 +82,14 @@ let cfg = config.documentation; in
       environment.systemPackages = [ pkgs.texinfoInteractive ];
       environment.pathsToLink = [ "/share/info" ];
       environment.extraOutputsToInstall = [ "info" ] ++ optional cfg.dev.enable "devinfo";
+      environment.extraSetup = ''
+        if [ -w $out/share/info ]; then
+          shopt -s nullglob
+          for i in $out/share/info/*.info $out/share/info/*.info.gz; do
+              ${pkgs.texinfo}/bin/install-info $i $out/share/info/dir
+          done
+        fi
+      '';
     })
 
     (mkIf cfg.doc.enable {