Web Forms :: .csv File Opens As An HTML Document - IE 7

In my application i allow users to download a csv file from the App. When a user clicks on the Download file link , they see the Download file dialog box. My problem is that some o f the files opens as an HTML document.This happens intermittently.

Response.Clear();
Response.ClearContent();
Response.ClearHeaders();
Response.ContentType =
"application/octet-stream"
;
Response.AppendHeader(
"Content-Disposition",
"attachment;
filename="
+ sf.FileName);
Response.OutputStream.Write(sf.Data, 0, sf.Data.Length);
Response.Flush();
Response.Close();

I tried Response.End() insted of Response.Flush(); but didnt help.