I have retested this again, on a platform version and on an updated 5.4.
On the platform installation, everything went ok and all tests for this passed.
However on a 5.4, as per conclusion on the discussions taken up above, I went to test with creation of an image content via the Public API.
I followed the example provided on the official documentation, and something strange about the storage paths of the binaries went on, that the rest of QA confirmed as well on their environments. I opened the issue EZP-27245 about this behaviour. It turns out this behaviour is unrelated to the new configurations made available on this jira, but rather about the API itself.
If there is some other way to test this (image content creation) not through Legacy other than through the public API, on a 5.4, can anyone let me know?
Ping Bertrand Dunogier, Jérôme Vieilledent.