```python
        import time

        from fastapi import FastAPI, Request

        app = FastAPI()


        @app.middleware("http")
        async def add_process_time_header(request: Request, call_next):
            start_time = time.time()
            response = await call_next(request)
            process_time = time.time() - start_time

        actual_response_class = response_class

    async def app(request: Request) -> Response:
        response: Union[Response, None] = None
        async with AsyncExitStack() as file_stack:
            try:
                body: Any = None
                if body_field:
                    if is_body_form:
                        body = await request.form()
                        file_stack.push_async_callback(body.close)