Error in Attachment file name

Hi below is my code

String fileName="XxxxxxxXxx_Sxxxxxxs.xls";(not real)
response.setContentType("application/vnd.ms-excel");
response.setHeader("Content-Disposition", "attachment; filename="+fileName);
return;

In IE7 the file name is getting appended with some junk charaters.
For IE8 and Firefox it is working correctly.
Did anybody know the reason?

From the trial and error method what I understand is ‘_S ‘is causing the problem.
If we change ‘_S’ to some other letters or append another letter in the end of filename this issue is not coming.

Is there any work around for this?Did anybody knows the reason for this?

Thanks