✨ Allow specifying max_field, max_files and max_part_size for routers and path operations

[YuriiMotov]

FastAPI currently allows configuring upload limits only at the application level (using middleware), which makes it difficult to apply different constraints to different endpoints.
In some cases you may want to specify different values for specific router or endpoint. Currently there is no such option.

The idea of this PR is to allow specifying such limits for routers and endpoints.

As an experiment, I implemented this for post method. If it's Ok then will implement for app, router and other methods

upload_file_router = APIRouter(form_max_files=100, max_part_size=1024*1024)


@upload_file_router.post(
    "/upload-multi", max_part_size=1024*1024*100,
    form_max_files=1000,
)
async def upload(files: List[UploadFile] = File(...)):
    pass


app = FastAPI(form_max_files=0, max_part_size=1024)

app.include_router(upload_file_router)

Related discussions:

• Max files/fields limit stands at 1000 anyway and breaks request #13557
• "Too many fields. Maximum number of fields is 1000" - How to change Starlette multipart-form default in FastAPI? #9634

[dolfinus]

Instead of modifying app.post() method, maybe it should be a part of annotation?

@app.post("/data")
async def handle_fiels(
  files: Annotated[List[UploadFile], Field(max_items=10), File(max_size="10MiB")]
): ...

[YuriiMotov]

Instead of modifying app.post() method, maybe it should be a part of annotation?

I think this is a bit different.. In my implementation max_field, max_files and max_part_size are used while parsing multipart data by Starlette. The idea of these parameters is to prevent the attacks when heavy requests are sent to cause "Out of memory" on server.
For now these parameters have defaults, but it's not easy to change these defaults for FastAPI app - people need to use middleware to call request.form() with custom parameters before validation