I have a Perl script that reads data from an Excel (`xls`) binary file. But the client that sends us these files has started sending us `XLSX` format files at times. I've updated the script to be able to read those as well. However, the client sometimes likes to name the `XLSX` files with an `.xls` extension, which currently confuses the heck outta my script since it uses the file name to determine which file type it is.

An `XLSX` file is a zip file that contains XML stuff. Is there a simple way for my script to look at the file and tell whether it's a zip file or not? If so, I can make my script go by that instead of just the file name.