.net core 文件下载

public async Task<IActionResult> Dowanload(string id,int numTypes)
        {
            try
            {
                using (var memory = new MemoryStream())
                {
                    await _document.DownloadToStreamAsync(id, memory);
                    var provider = new FileExtensionContentTypeProvider();
                    var memi = provider.Mappings[".txt"];
                    return File(memory, memi, ((NumScreeningTypes)numTypes) + ".txt");
                }
            }
            catch (Exception ex)
            {
                return null;
            }
            
        }