MyController.cs

        [HttpGet("/q1")]
        public IActionResult Get()
        {
            return new FileCallbackResult(new MediaTypeHeaderValue("application/octet-stream"),
                async (stream, context) =>
                {
                    context.HttpContext.Response.Body.Flush();

                    await Task.Delay(4000);
                    var data = Encoding.UTF8.GetBytes("asdaasdfasdfasdfasdfasdsdasdasd");
                    stream.Write(data, 0, data.Length);
                })
            {
                FileDownloadName = "README.md"
            };
        }