搞了好久也没有搞通，网上有很多事例都是一样的：IE支持的文件就会直接打开，而其它的像ZIP之类的文件不识别，就会弹出下载或打开的对话框。
突然一想是不是有什么文件筛选的问题，果然在写文件之前将Filter清除就可以像附件一样使用了。

VB.net:
  Dim filename As String =  "a.txt"
 
        If filename <> "" Then
 
            Dim path As String =  Server.MapPath(filename)
 
            Dim file As System.IO.FileInfo =  New System.IO.FileInfo(path)
 
            If file.Exists Then
 
                Response.Clear()
 
                Response.AddHeader("Content-Disposition", "attachment; filename=" + file.Name)
 
                Response.AddHeader("Content-Length", file.Length.ToString())
 
                Response.ContentType = "application/octet-stream"
 
                Response.Filter.Close()
 
                Response.WriteFile(file.FullName)
 
 
                Response.End()
 
            Else
 
                Response.Write("This file does not exist.")
 
            End If
 
        End If

----------------------------------------------------------------
c#:

   string filename = "a.txt";

        if (filename != "")
        {

            string path = Server.MapPath(filename);

            System.IO.FileInfo file = new System.IO.FileInfo(path);

            if (file.Exists)
            {

                Response.Clear();

                Response.AddHeader("Content-Disposition", "attachment; filename=" + file.Name);

                Response.AddHeader("Content-Length", file.Length.ToString());

                Response.ContentType = "application/octet-stream";

                Response.Filter.Close();

                Response.WriteFile(file.FullName);

              
                Response.End();

            }

            else
      &nbs