Hello @Michael Ryan
As per our documentation
The package file must be .zip formatted.
This is because it uses Kudu's zipdeploy and only supports .zip files
https://github.com/projectkudu/kudu/blob/master/Kudu.Services/Zip/ZipController.cs
I hope this helps provide you with the information you need. If it does, please make sure to mark the question as answered so it helps other people in future.
Kind Regards
Alistair