FileInfo fi = new FileInfo( Server.MapPath( "~" ) + "\\downloads\\" + fileName );
if (fi != null && fi.Exists)
{
	int BUFF_LENGTH = 10000;
	int length = Convert.ToInt32( fi.Length > BUFF_LENGTH ? BUFF_LENGTH : fi.Length );
	long dataToRead = fi.Length;
	using (FileStream fs = fi.OpenRead())
		try
		{
			Response.BufferOutput = false;
			Response.ContentType = "application/octet-stream";
			Response.AddHeader( "content-disposition", "attachment; filename=" + fileName );
			byte[] fileContent = null;
			while (dataToRead > 0)
				if (Response.IsClientConnected)
				{
					fileContent = new byte[length];

					length = fs.Read( fileContent, 0, BUFF_LENGTH );

					Response.OutputStream.Write( fileContent, 0, fileContent.Length );
					Response.Flush();
					dataToRead -= length;
				}
				else
					dataToRead = -1;
		}
		finally
		{
			Response.Close();
		}
}

<%@ Page Language="C#" EnableSessionState="ReadOnly" %>