Are you really sure it is a new feature and not a bug fix?
Actually, version 3.1 was developed as a "vast improvement" to the former modifications (MODs).
So I think it is a mere bug fix because they just forgot to integrate the upload button on development.https://www.phpbb.com/extensions/installing/
With extensions, you no longer have to modify a single line of code in phpBB.
Extensions are simply uploaded to your board and enabled inside the ACP.
Updating, disabling and removing extensions is equally as easy to perform.
It is a step in the right direction....
There is already a validated extension available. Just integrate it into the 3.2 package...
...or change the upload path in AutoMOD and put it into the phpBB 3.2 standard package.