Web Forms :: Setting Up Date / Time Stamp For Downloaded File?

I am trying to download byte array (l_attchmntToDownload) using the following code:

try{
Response.Clear();
Response.AppendHeader("Content-Disposition", "attachment; filename="+l_attchmntFileSpec);
Response.ContentType = "application/octet-stream";
Response.BinaryWrite(l_attchmntToDownload);
Response.End();
}
catch (Exception Ex)
{
throw Ex;
}

Is it possible to supply Date/Time of modification for the downloaded file?

I tested "Last-Modified" HTTP header but the file gets saved with the current Date/Time only.