diff --git a/nixos/tests/mattermost.nix b/nixos/tests/mattermost.nix
index 04df954eef1a6..a39033ced31d6 100644
--- a/nixos/tests/mattermost.nix
+++ b/nixos/tests/mattermost.nix
@@ -52,6 +52,16 @@ import ./make-test-python.nix (
           ];
       };
       immutable = makeMattermost {
+        package = pkgs.mattermost.overrideAttrs (prev: {
+          webapp = prev.webapp.overrideAttrs (prevWebapp: {
+            # Ensure that users can add patches.
+            postPatch =
+              prevWebapp.postPatch or ""
+              + ''
+                substituteInPlace channels/src/root.html --replace-fail "Mattermost" "Patched Mattermost"
+              '';
+          });
+        });
         mutableConfig = false;
         extraConfig.SupportSettings.HelpLink = "https://search.nixos.org";
       };
@@ -94,6 +104,8 @@ import ./make-test-python.nix (
         ## Mutable node tests ##
         mutable.wait_for_unit("mattermost.service")
         mutable.wait_for_open_port(8065)
+        mutable.succeed("curl -L http://localhost:8065/index.html | grep '${siteName}'")
+        mutable.succeed("curl -L http://localhost:8065/index.html | grep 'Mattermost'")
 
         # Get the initial config
         mutable.succeed("${expectConfig ''.AboutLink == "https://nixos.org" and .HelpLink == "https://search.nixos.org"''}")
@@ -110,6 +122,8 @@ import ./make-test-python.nix (
         ## Mostly mutable node tests ##
         mostlyMutable.wait_for_unit("mattermost.service")
         mostlyMutable.wait_for_open_port(8065)
+        mostlyMutable.succeed("curl -L http://localhost:8065/index.html | grep '${siteName}'")
+        mostlyMutable.succeed("curl -L http://localhost:8065/index.html | grep 'Mattermost'")
 
         # Get the initial config
         mostlyMutable.succeed("${expectConfig ''.AboutLink == "https://nixos.org"''}")
@@ -126,6 +140,8 @@ import ./make-test-python.nix (
         ## Immutable node tests ##
         immutable.wait_for_unit("mattermost.service")
         immutable.wait_for_open_port(8065)
+        # Since we patched it, it doesn't replace the site name at runtime anymore
+        immutable.succeed("curl -L http://localhost:8065/index.html | grep 'Patched Mattermost'")
 
         # Get the initial config
         immutable.succeed("${expectConfig ''.AboutLink == "https://nixos.org" and .HelpLink == "https://search.nixos.org"''}")
@@ -143,6 +159,8 @@ import ./make-test-python.nix (
         ## Environment File node tests ##
         environmentFile.wait_for_unit("mattermost.service")
         environmentFile.wait_for_open_port(8065)
+        environmentFile.succeed("curl -L http://localhost:8065/index.html | grep '${siteName}'")
+        environmentFile.succeed("curl -L http://localhost:8065/index.html | grep 'Mattermost'")
 
         # Settings in the environment file should override settings set otherwise
         environmentFile.succeed("${expectConfig ''.AboutLink == "https://nixos.org"''}")
diff --git a/pkgs/by-name/ma/mattermost/package.nix b/pkgs/by-name/ma/mattermost/package.nix
index 87bbe84027130..61036a0fdb9ef 100644
--- a/pkgs/by-name/ma/mattermost/package.nix
+++ b/pkgs/by-name/ma/mattermost/package.nix
@@ -6,7 +6,6 @@
   nix-update-script,
   npm-lockfile-fix,
   fetchNpmDeps,
-  diffutils,
   jq,
   nixosTests,
 }:
@@ -94,7 +93,7 @@ buildGoModule rec {
       runHook preInstall
 
       mkdir -p $out
-      cp -r channels/dist/* $out
+      cp -a channels/dist/* $out
 
       runHook postInstall
     '';
@@ -123,11 +122,20 @@ buildGoModule rec {
   ];
 
   postInstall = ''
-    mkdir -p $out/{client,i18n,fonts,templates,config}
-    cp -r ${webapp}/* $out/client/
-    cp -r ${src}/server/i18n/* $out/i18n/
-    cp -r ${src}/server/fonts/* $out/fonts/
-    cp -r ${src}/server/templates/* $out/templates/
+    shopt -s extglob
+    mkdir -p $out/{i18n,fonts,templates,config}
+
+    # Link in the client and copy the language packs.
+    ln -sf $webapp $out/client
+    cp -a $src/server/i18n/* $out/i18n/
+
+    # Fonts have the execute bit set, remove it.
+    cp --no-preserve=mode $src/server/fonts/* $out/fonts/
+
+    # Don't copy the Makefile.
+    cp -a $src/server/templates/!(Makefile) $out/templates/
+
+    # Generate the config.
     OUTPUT_CONFIG=$out/config/config.json \
       go run -tags production ./scripts/config_generator
   '';