SWT OleClientSite: How to load a no-real xls file in Excel?

Question

I have an Excel file in OfficeML format, MyData.xls, but it's not really a XLS format but a HTMLfile with a XLS extension. Since I upgraded to Office 2007 from Office 2003 I get a warning message saying that the file content does not match the file extension. It seems that OfficeML now check if the file is a pure XLS file or not.

In my application I use OleClientSite to display the file in an OleFrame object. If I change the file extension to 'HTML' then Excel is not started. If I leave the extension as 'XLS' then I get the above warning message.

How can I force the file with the 'html' extension to be opened in the OleFrame using Excel?