Fix static asset serving in Modal app

main.py

@@ -19,7 +19,7 @@ app = modal.App("formex-viewer", image=app_image)
 )
 @modal.asgi_app()
 def fastapi_app():
-    import fastapi.staticfiles
+    import fastapi
 
     from formex_viewer.server import app as formex_app
 
@@ -28,7 +28,7 @@ def fastapi_app():
     @formex_app.get("/{path:path}")
     async def frontend_handler(path: str):
         fp = assets_path / path
-        if not fp.exists():
+        if not fp.exists() or not fp.is_file():
             fp = assets_path / "index.html"
         return fastapi.responses.FileResponse(fp)