Hey guys,
i have a couple of excel .XLSX files that i have been trying to open but i haven’t had any luck with then excel says “excel cannot open filename.xlsx because the file format or file extension is not valid. verify that the file has not been corrupted and that the file extension matches the format of the file”
You could just try opening it with Libre Office which is completely free and very good at fixing this sort of thing.
Read here: http://www.excelforum.com/excel-general/1027831-problem-opening-a-xls-file.html