diff --git a/.github/workflows/lint.yaml b/.github/workflows/lint.yml
similarity index 67%
rename from .github/workflows/lint.yaml
rename to .github/workflows/lint.yml
index 5596ca5..736aff0 100644
--- a/.github/workflows/lint.yaml
+++ b/.github/workflows/lint.yml
@@ -47,3 +47,21 @@ jobs:
       - uses: zizmorcore/zizmor-action@cc914d7f3750a2d13d75c7f184a1060aa0e9d482 # v0.6.4
         with:
           advanced-security: false
+
+  extensions:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        with:
+          persist-credentials: false
+
+      - name: GitHub Actions files end in .yml, other YAML files in .yaml
+        run: |
+          wrong=$(
+            git ls-files '.github/*.yaml' 'action.yaml' '*/action.yaml'
+            git ls-files '*.yml' ':!.github/*' ':!action.yml' ':!*/action.yml'
+          )
+          if [ -n "$wrong" ]; then echo "$wrong"; exit 1; fi
diff --git a/.github/workflows/required.yaml b/.github/workflows/required.yml
similarity index 88%
rename from .github/workflows/required.yaml
rename to .github/workflows/required.yml
index ed22d38..8b16360 100644
--- a/.github/workflows/required.yaml
+++ b/.github/workflows/required.yml
@@ -7,12 +7,12 @@ on:
 permissions: {}
 jobs:
   lint:
-    uses: ./.github/workflows/lint.yaml
+    uses: ./.github/workflows/lint.yml
     permissions:
       contents: read
 
   test:
-    uses: ./.github/workflows/test.yaml
+    uses: ./.github/workflows/test.yml
     permissions:
       contents: read
 
diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yml
similarity index 100%
rename from .github/workflows/test.yaml
rename to .github/workflows/test.yml