upload file problem when not logged as admin

hi,
i run a modx 0.9.6 website.
i can upload files or images when i’m logged as the administrator, but not when i’m logged as a manager user:
i created a role called "webmaster", where users can manage documents. When i try to upload a file, i can’t see files that are already uploaded in the same directory, and i get the error "disallowed file type" for any kind of file i try to upload as a "webmaster", but I get no problem as the "admin" (and the file types are well specified in the configuration tab).
I wonder if it’s a problem of rights, but i just can change the rights even via ftp (which is quite strange)!
please does anybody have an idea?
thanks!

Each user has a section for file management rights, in the "User" tab.

thanks!