Dear Leslie,
We are glad to help you and may need some more information for troubleshooting.
Do you mean that the file becomes filename.html after you rename the PowerPoint file and change the extension to .zip?
If yes, may I know whether the issue happens with other types of Office files or non-Office files? Does the issue occur only with PowerPoint files?
If it is convenient, I suggest you right-click on the file>Compress the file to see if it can become the .zip file.
Welcome to share any updates at your convenience.
Thank you for your time and effort.
Sincerely
Cliff | Microsoft Community Moderator