fix: support mounting StaticFiles on APIRouter and propagate via include_router

- Override APIRouter.mount() to only allow StaticFiles; raise FastAPIError
  for any other ASGI app with a clear error message
- Add APIRouter._mount() as internal method used by include_router()
  and FastAPI.mount() to bypass the StaticFiles-only restriction
- Override FastAPI.mount() to call self.router._mount() directly
- Propagate StaticFiles mounts in include_router() combining
  prefix + router.prefix + route.path correctly

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

@@ -74,6 +74,7 @@ from starlette.routing import (
     get_name,
 )
 from starlette.routing import Mount as Mount  # noqa
+from starlette.staticfiles import StaticFiles
 from starlette.types import AppType, ASGIApp, Lifespan, Receive, Scope, Send
 from starlette.websockets import WebSocket
 from typing_extensions import deprecated
@@ -993,6 +994,20 @@ 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.
+        Will raise a FastAPIError exception if app is not an instance of StaticFiles.
+        """
+        if not isinstance(app, StaticFiles):
+            raise FastAPIError(
+                "APIRouter does not support mounting ASGI applications other than StaticFiles."
+            )
+        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,
@@ -1489,6 +1504,9 @@ class APIRouter(routing.Router):
                 self.add_websocket_route(
                     prefix + route.path, route.endpoint, name=route.name
                 )
+            elif isinstance(route, routing.Mount):
+                if isinstance(route.app, StaticFiles):
+                    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_apirouter_mount.py

@@ -0,0 +1,81 @@
+"""Tests for mounting StaticFiles under APIRouter (issue #10180)."""
+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
+
+
+def test_mount_staticfiles_under_router_with_prefix(tmp_path: Path) -> None:
+    """StaticFiles mounted on APIRouter with prefix should be accessible."""
+    static_file = tmp_path / "hello.txt"
+    static_file.write_text("hello world")
+
+    app = FastAPI()
+    api_router = APIRouter(prefix="/api")
+
+    @api_router.get("/app")
+    def read_main() -> dict:
+        return {"message": "Hello World from main app"}
+
+    api_router.mount("/static", StaticFiles(directory=tmp_path), name="static")
+    app.include_router(api_router)
+
+    client = TestClient(app)
+
+    # Regular route should still work
+    r = client.get("/api/app")
+    assert r.status_code == 200
+
+    # StaticFiles should be accessible under router prefix
+    r = client.get("/api/static/hello.txt")
+    assert r.status_code == 200
+    assert r.text == "hello world"
+
+
+def test_mount_staticfiles_under_router_without_prefix(tmp_path: Path) -> None:
+    """StaticFiles mounted on APIRouter without prefix should be accessible."""
+    static_file = tmp_path / "test.txt"
+    static_file.write_text("test content")
+
+    app = FastAPI()
+    router = APIRouter()
+    router.mount("/static", StaticFiles(directory=tmp_path), name="static")
+    app.include_router(router)
+
+    client = TestClient(app)
+
+    r = client.get("/static/test.txt")
+    assert r.status_code == 200
+    assert r.text == "test content"
+
+
+def test_mount_staticfiles_with_include_router_prefix(tmp_path: Path) -> None:
+    """include_router prefix + router prefix + mount path should all combine."""
+    static_file = tmp_path / "file.txt"
+    static_file.write_text("combined prefix")
+
+    app = FastAPI()
+    router = APIRouter(prefix="/api")
+    router.mount("/static", StaticFiles(directory=tmp_path), name="static")
+    app.include_router(router, prefix="/v1")
+
+    client = TestClient(app)
+
+    r = client.get("/v1/api/static/file.txt")
+    assert r.status_code == 200
+    assert r.text == "combined prefix"
+
+
+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()
+
+    with pytest.raises(
+        FastAPIError,
+        match="APIRouter does not support mounting ASGI applications other than StaticFiles.",
+    ):
+        router.mount("/sub", sub_app)