Thanks for your kind and detailed answer.
As you can see in my first post inside this current topic, I did set the Attachment Storage Settings to the filesystem (via the plugin).
And yes, the plugin is Enabled.
I also don't have any old attachments because this is a fresh installation (I just discovered the amazing osTicket project two days ago).
The thing is that, after uploading new attachments I just went to the attachments folder (out of curiosity, to see how files are structured).
And to my surprise, inside that folder, there aren't any attachmente files, but some folders (see screenshot of the containing folders on my post above) and a .keep file.
What can I do to provide more information about this?
Thanks for your time.