The default subsystem that's enabled on installation is AzureOnPrem. This ensures that the new AMP version is compatible with a previous installation.
You can change the subsystem used by overwriting the global variable filecontentstore.subsystem.name, for example:
filecontentstore.subsystem.name=Azure
Note
Do not switch to a pure Azure content store from AzureOnPrem if binaries have already been saved on the file system.