Is it because they uploaded the files to their user file root directories? /media/users/xyz/ ?

Cheat, create a dummy blog that only admin has access to and change it's media folder to /media/users ;)

¥

Is it because they uploaded the files to their user file root directories? /media/users/xyz/ ?

Yes
(Hello Francois, in which country are you right now ?)

Cheat, create a dummy blog that only admin has access to and change it's media folder to /media/users

I will try that

[edit: It works. Thanks for the idea]

Cheat: good to see files, bad to modify them, since b2evo won't know it's the same file. In case of linked files, it will lead to side effects.

Well, so I need a other way to access this file.

Yep, I put it on the todo list for the admin to see all user roots. Could be messy when there are many users :/

How many users do you have ?

4 authors on one blog, 2 on a second blog, 1 on a third blog

Yep, with 4 t shouldn't be a problem :)