Okay so guys, I did it. Here is what I did, in case someone found this thread from Google and has the same problem.
1st: Check what your folders/users are. I used putty and did this:
- cd /
- ls -la (Just in order to check all the folders and who has the rights on them (normally, you'll see only "root")
- cut -d: -f1 /etc/passwd (You also need to check, with this. If you have "extplorer" as user. if you installed the plugin, you must have it)
2nd: Create a new folder that will be used by extplorer. I created in the main directory a folder named "partage" (share in french), then:
- mkdir partage (creates the folder)
- sudo chown extplorer: partage (this line will give "extplorer" the full power on the folder "partage" (or whatever you named it). Someone on askubuntu said you could use "setfacl -m u:username:rwx myfolder" so it just sets permissions for specific users, without changing the ownership of the directory. But I didn't test it.
3nd: Now you can create/delete/modify anything inside that "partage" folder, and only into this one. For exemple, you can create inside this one a folder name "files"
4nd: You can now create a new user (for example "roberto") and give him "/partage/files" in "home directory" (or "home folder", I don't know the english translation), and let "http://localhost" as "root path". I gave my users "Modify & Change password"
5nd: Open the port 443 (I created a certificate so I can use httpS) and then you can access it, from outside, with the adress htt.ps://yournoipadress.ddns.net/extplorer/index.php
.
6nd: You can now connect with roberto / hispassword, he'll only see what's inside "/partage/files" and will be able to create, delete, download, upload.. 
I'm just trying to help with what I did. I don't know if what I did was the good way or whatever. So like everyone on Internet i'd say "Do it on your own risk" 
If some people read this and want to add some things, or say if I did something wrong, please do it
But i'm glad it FINALLY works for me!
