See https://github.com/microsoft/azure-spring-boot/issues/926 and https://github.com/spring-projects/spring-boot/issues/22278 for background. Azure does not allow . characters in the name of a secret file so you have to jump through a renaming hoop to add the .properties or .yaml suffix.
Comment From: philwebb
We'll look at supporting a new syntax for this. Something like spring.config.location=/etc/application[.properties] where the [.properties] isn't part of the filename, but just a hint for the content type.