The sendFile method itself cannot convert html content (or any other) to xls file. You need to do additional work to make it happen like with the help of extension.
Apparently Excel can read html files with basic table tags (didn’t know that before!). Anyway - this has nothing to do with sendFile method but… One thing you can try is to set third sendFile parameter which is $mimeType to one of the types recognized by Excel http://stackoverflow.com/questions/974079/setting-mime-type-for-excel-document although I’m not sure this will help with the error.