"Save as" option save Excel file as HTML by default?

I've written a simple reporting application which will generate a report in Excel format. Upon completion the page (ReportList.jsp) will then forward a URL to another page (ReportSuccess.jsp) where user have to right click and choose "Save as" to retrieve the report.

On the development environment (XP SP2 with MS Office installed) there is no problem encountered. However, when I ported over the codes to the server (Windows Server 2003 with Open Office installed), when user right click and "Save as" the type is changed to HTML by default.

I've set the file association of .XLS files to be opened by Calc but still that does not solve it. Any other idea?

Additional info: When I right click on the link to view the properties it showed up correctly as "http://<myIP>:8185/SnetReport/REPORT/Gymboree.xls"