I am curious about what You mean by this, exactly:
wrote: the file could not be opened due to file format or extension
If I were to save some excel file and renamed the extension from ".xlsx" to, say, ".xslx", and then tried to open it in excel, the error message would also look quite similar. Also, in addition to the suggestions above of trying .zip as an extension, you may want to try opening the file in a text editor (e.g., by renaming it to ".txt").