方式一：

public FileStreamResult DownFile(string filePath, string fileName)
        {
            string absoluFilePath = Server.MapPath(System.Configuration.ConfigurationManager.AppSettings["AttachmentPath"] + filePath);
            return File(new FileStream(absoluFilePath, FileMode.Open), "application/octet-stream", Server.UrlEncode(fileName));
        }

方式二：

public ActionResult DownFile(string filePath, string fileName)
        {
            filePath = Server.MapPath(System.Configuration.ConfigurationManager.AppSettings["AttachmentPath"] + filePath);
            FileStream fs = new FileStream(filePath, FileMode.Open);
            byte[] bytes = new byte[(int)fs.Length];
            fs.Read(bytes, 0, bytes.Length);
            fs.Close();
            Response.Charset = "UTF-8";
            Response.ContentEncoding = System.Text.Encoding.GetEncoding("UTF-8");
            Response.ContentType = "application/octet-stream";

            Response.AddHeader("Content-Disposition", "attachment; filename=" + Server.UrlEncode(fileName));
            Response.BinaryWrite(bytes);
            Response.Flush();
            Response.End();
            return new EmptyResult();

        }

View调用：

<a href="/Document/DownFile?filePath=@item.Value&fileName=@item.Key">下载</a>