public FileResult Down()
        {
　　　　　　　　return File(xxx,xxx,xxx);
        }

var stream = System.IO.File.OpenRead(“F:\a.txt”);  //創建文件流

return File(stream, "text/plain", “b.txt");

//引入
using System.IO;

var contentype = MimeMapping.GetMimeMapping(fileName);  　　　　//獲取文件類型

 public FileResult DownSH(string DownM)  //要下載文件碼
        {
            var path = _sqlContext.jexusSqls.FirstOrDefault(a => a.DownM == DownM.ToString()).FilePath;  //在數據庫查找此文件碼的文件地址
            if (!System.IO.File.Exists(path)) return null;

            var stream = System.IO.File.OpenRead(path);  //創建文件流

            return File(stream, "application/x-sh", DownM+".sh");
        }