On my website I’ve got a number of reports that are generated in html and then put into a file with a .xls extension. The file is then made available to download on the web page. This worked ok up until a couple of weeks ago. It seems an update to Excel means that if you open a file with a .xls format it’s expecting an excel file, it won’t accept html content.

I can use an .htm extension etc however that’ll cause the file to open in a browser and I've no control over the user's browsers or PC's.
Any idea how I can keep these reports in HTML format but ensure they open in excel when the file attachment is clicked on the web page?

Thanks in advance

Stephen