參考連結：

遇到的問題：

1. 按 參考1 中測試，下載檔案檔名總是變成方法名(DownloadFile)，並且沒有副檔名，儲存後改副檔名可正常檢視。
2. 參考 連結3 測試無效，未解決下載問題。
3. 參考 連結2，問題解決。
   程式碼實現如下：

檔案上傳

[HttpPost]
public async Task<ActionResult> UploadFile()
{
   _folder = $@"{env.WebRootPath}\Upload";//env 可通過Controller的引數傳入，並取得：IHostingEnvironment _env
    string controller = RouteData.Values["controller"
 
].ToString();
    var dirPath = _folder + $"/Template/{controller}";
    if (!Directory.Exists(dirPath))
    {
        Directory.CreateDirectory(dirPath);
    }

    var size = Request.Form.Files.Sum(f => f.Length);
    IFormFile file = Request.Form.Files.FirstOrDefault();
    if (file.Length > 0
 
)
    {
        var filePath = $@"{dirPath}\{file.FileName}";
        using (var stream = new FileStream(filePath, FileMode.Create))
        {
            await file.CopyToAsync(stream);//寫入到伺服器。
        }
    }
    _controllername = RouteData.Values["controller"].ToString();
    string Id = Request.Form["Id"
 
].First();
    var path = $"/api/{_controllername}/Detail/{Id}";
    var data = _excelTemplateDocService.Get(path).Result.FirstOrDefault();

    if(data!=null)
    {
        data.ExcelPath = file.FileName;
        path = $"/api/{_controllername}";
        await _excelTemplateDocService.Post(path, data);
    }
    else
    {
        return BadRequest("ExcelPath未更新!");
    }
    return Ok(new { code = 200, message = "success" });
}

檔案下載

[HttpGet]
public async Task<IActionResult> DownloadFile(string fileName)
{
    if (string.IsNullOrEmpty(fileName))
    {
        return NotFound();
    }
    string controller = RouteData.Values["controller"].ToString();
    var path = _folder + $"/Template/{controller}/{fileName}";
    var memoryStream = new MemoryStream();
    using (var stream = new FileStream(path, FileMode.Open))
    {
        await stream.CopyToAsync(memoryStream);
    }
    memoryStream.Seek(0, SeekOrigin.Begin);
    //檔名必須編碼，否則會有特殊字元(如中文)無法在此下載。
    string encodeFilename = msSystem.Web.HttpUtility.UrlEncode(fileName, msSystem.Text.Encoding.GetEncoding("UTF-8"));
    Response.Headers.Add("Content-Disposition", "attachment; filename=" + encodeFilename);
    return new FileStreamResult(memoryStream, "application/octet-stream");//檔案流方式，指定檔案流對應的ContenType。
}