将下载另存为PDF

Question

我需要在DownloadFile方法中对此代码进行编程，以自动选择应为PDF类型的文件。

当前，我必须在索引中的下载链接上单击“保存”，然后选择打开文件的文件类型，然后保存文件。如果我不选择文件类型，则会将其保存为未知文件。

请先完成此操作，谢谢。

    /// <summary>
    /// Retrive Image from database 
    /// </summary>
    /// <param name="id"></param>
    /// <returns></returns>
    public ActionResult RetrieveImage(int id)
    {
        byte[] cover = GetImageFromDataBase(id);

        if (cover != null)
        {
            return File(cover, "application.pdf");
        }
        else
        {
            return null;
        }
    }
    /// <summary>
    /// 
    /// </summary>
    /// <param name="Id"></param>
    /// <returns></returns>
    public byte[] GetImageFromDataBase(int Id)
    {
        var q = from temp in db.Contents where temp.ID == Id select temp.Image;
        byte[] cover = q.First();

        return cover;
    }



    [HttpPost]
    public FileResult DownloadFile(int? fileId)
    {
        byte[] bytes;
        string fileName, Description;
        string constr = System.Configuration.ConfigurationManager.ConnectionStrings["dbContext"].ConnectionString;
        using (SqlConnection con = new SqlConnection(constr))
        {
            using (SqlCommand cmd = new SqlCommand())
            {
                cmd.CommandText = "SELECT Title, Description, Image FROM [Contents] WHERE Id=@Id";
                cmd.Parameters.AddWithValue("@ID", fileId);
                cmd.Connection = con;
                con.Open();
                using (SqlDataReader sdr = cmd.ExecuteReader())
                {
                    sdr.Read();
                    bytes = (byte[])sdr["Image"];
                    Description = sdr["Description"].ToString();
                    fileName = sdr["Title"].ToString();
                }
                con.Close();
            }
        }

        return File(bytes, Description, fileName);
    }


    public byte[] ConvertToBytes(HttpPostedFileBase image)
    {
        byte[] imageBytes = null;
        BinaryReader reader = new BinaryReader(image.InputStream);
        imageBytes = reader.ReadBytes((int)image.ContentLength);
        return imageBytes;
    }