BaseHTTPMiddleware causes form data loss when parsing with custom limits

Using BaseHTTPMiddleware to intercept requests and call request.form() with custom max_part_size results in complete loss of form data. The form data is not passed through to downstream handlers after being parsed in the middleware.

Do not use BaseHTTPMiddleware for form parsing interception. Instead, use alternative approaches: create a custom request class following FastAPI's custom request documentation, use only the Request dependency with custom form constraints (though this won't work in all cases), or create a custom middleware inspired by base HTTP middleware that properly forwards the stream or caches it. Reference discussion #13739 for more details on BaseHTTPMiddleware limitations.