store mimeExtension.js file in artifacts for debugging

Author: emilykl

.github/workflows/check-js-build.yml

@@ -71,7 +71,9 @@ jobs:
         else
         echo "✅ Build artifacts match expected output (ignoring known 'load' hash in package.json)."
         fi
-
+    - name: Move js/lib/mimeExtension.js to plotly/labextension/mimeExtension.js
+      run: |
+        mv js/lib/mimeExtension.js plotly/labextension/mimeExtension.js
     - name: Store the build artifacts from plotly/labextension
       uses: actions/upload-artifact@v4
       if: failure()