C # about downloading document operations functions

<summary>
Letter of credit Material
</summary>
<param name= "FileName" ></param>
public void DownLoad (string fileName = "Financial trading platform-search House Financial management background usage Instructions v1.1.pdf")
{
string browser = Request.UserAgent.ToUpper ();
FileStream FileStream = new FileStream (Server.MapPath ("/manager/attach/" + fileName), FileMode.Open, System.IO.FileAccess.Read, System.IO.FileShare.Read);
Long filesize = FileStream. Length;
Allow the client browser to correctly identify the type and file size of this file
System.IO.Path.GetFileName (Server.MapPath ("/manager/attach/" + fileName). ToLower ();
Response.ContentType = "Application/octet-stream";
if (browser. Contains ("FIREFOX") = = True)
{
filename = filename;
}
Else
{
filename = httputility.urlencode (filename, System.Text.Encoding.UTF8);

}
Response.AddHeader ("Content-disposition", "attachment; Filename=\ "+ fileName +" \ ";");
Response.AddHeader ("Content-length", FileSize. ToString ());
Sending data from a file to the client
byte[] Filebuffer = new Byte[filesize];
FileStream. Read (Filebuffer, 0, (int) filesize);
Response.BinaryWrite (Filebuffer);

FileStream. Close ();
Response.End ();
}

C # about downloading document operations functions