fix: support mounting StaticFiles to APIRouter, raise error for other apps

- Override APIRouter.mount() to only allow StaticFiles; raise FastAPIError
  for any other ASGI app with a clear message directing users to mount
  sub-applications on the root FastAPI instance instead
- Add APIRouter._mount() as internal bypass used by include_router()
  and FastAPI.mount()
- Override FastAPI.mount() to call self.router._mount() directly,
  bypassing the StaticFiles-only restriction on APIRouter
- Propagate StaticFiles mounts in include_router() via _mount()

Fixes #10180

fastapi/applications.py

@@ -1133,6 +1133,9 @@ class FastAPI(Starlette):
             scope["root_path"] = self.root_path
         await super().__call__(scope, receive, send)
 
+    def mount(self, path: str, app: ASGIApp, name: str | None = None) -> None:
+        self.router._mount(path, app=app, name=name)
+
     def add_api_route(
         self,
         path: str,

fastapi/routing.py

@@ -994,6 +994,26 @@ class APIRouter(routing.Router):
         self.default_response_class = default_response_class
         self.generate_unique_id_function = generate_unique_id_function
 
+    def mount(self, path: str, app: ASGIApp, name: str | None = None) -> None:
+        """
+        Mount a `StaticFiles` instance at the given path.
+
+        Raises `FastAPIError` if `app` is not an instance of `StaticFiles`,
+        since mounting arbitrary ASGI sub-applications under an `APIRouter`
+        is not supported. Sub-applications should be mounted directly on
+        the root `FastAPI` instance instead.
+        """
+        if not isinstance(app, StaticFiles):
+            raise FastAPIError(
+                "APIRouter does not support mounting ASGI applications other than "
+                "StaticFiles. Mount sub-applications directly on the root FastAPI "
+                "instance instead."
+            )
+        self._mount(path=path, app=app, name=name)
+
+    def _mount(self, path: str, app: ASGIApp, name: str | None = None) -> None:
+        super().mount(path=path, app=app, name=name)
+
     def route(
         self,
         path: str,
@@ -1492,7 +1512,7 @@ class APIRouter(routing.Router):
                 )
             elif isinstance(route, routing.Mount):
                 if isinstance(route.app, StaticFiles):
-                    self.mount(prefix + router.prefix + route.path, route.app, name=route.name)
+                    self._mount(prefix + router.prefix + route.path, route.app, name=route.name)
         for handler in router.on_startup:
             self.add_event_handler("startup", handler)
         for handler in router.on_shutdown:

tests/test_router_mount.py

@@ -3,6 +3,7 @@ from pathlib import Path
 
 import pytest
 from fastapi import APIRouter, FastAPI
+from fastapi.exceptions import FastAPIError
 from fastapi.staticfiles import StaticFiles
 from fastapi.testclient import TestClient
 
@@ -68,27 +69,13 @@ def test_mount_staticfiles_with_include_router_prefix(tmp_path: Path) -> None:
     assert r.text == "combined prefix"
 
 
-def test_mount_non_staticfiles_app_is_ignored() -> None:
-    """Mounting a non-StaticFiles ASGI app on APIRouter should be silently ignored
-    (not propagated to the main app) to preserve existing behavior."""
-    app = FastAPI()
-    api_router = APIRouter(prefix="/api")
-
-    subapi = FastAPI()
-
-    @subapi.get("/sub")
-    def read_sub() -> dict:
-        return {"message": "sub"}
-
-    api_router.mount("/subapi", subapi)
-    app.include_router(api_router)
-
-    client = TestClient(app)
-
-    # Non-StaticFiles sub-app mount is not propagated — returns 404
-    r = client.get("/api/subapi/sub")
-    assert r.status_code == 404
+def test_mount_non_staticfiles_app_raises_error() -> None:
+    """Mounting a non-StaticFiles ASGI app on APIRouter should raise FastAPIError."""
+    router = APIRouter()
+    sub_app = FastAPI()
 
-    # Regular routes are unaffected
-    # (no routes defined here, but include_router itself should not crash)
-    assert app is not None
+    with pytest.raises(
+        FastAPIError,
+        match="APIRouter does not support mounting ASGI applications other than StaticFiles",
+    ):
+        router.mount("/sub", sub_app)