Problem with file downloads -- zip files are invalid

I would like to download all the emails (several years of them) from the online (web) version of Xfinity for my primary and secondary accounts. However, when I select a group of emails, then click the File Download option, the resulting zip files are all invalid format and unreadable on my Windows 10 PC. What am I doing wrong?

If it helps to know, I do not have a local email client (e.g., Outlook) mapped; I only use the web interface to access the emails on the server.