How to set content type before opening PopUp window in Javascript?

I am using following code to open PopWindow as a excel sheet:::---


wind=window.open(open_report,'popup','toolbar=yes, scrollbars=yes,resizable=yes,top=0,left=0,width=70 0,height=700,menuBar=yes');


I want to set ----------
response.setContentType( application/vnd.ms-excel );
so that opened window will show excel sheet.

Can anyone help me for telling
where to set content type.????
I tried to set it in java class also tried in JSP in which I am opening pop up.But after opening pop up its showing data with unreadable format.