Redmine

We have following problem: The user group "NotMember" with the role "Reporter" is added to a project. The role "Reporter" can only create new tickets. As you create a new ticket you can upload files. But this isn't working. You can only upload files if you are manualy added as a member to the project with the role "Reporter".
Is it really necessary to add every user manualy as a member to upload files in the ticket system ? It would help a lot if user of the group "NotMember" could upload files to the ticket system. If not then how could we hide the upload button ?

Our user authentication is done with LDAP.

Environment:
Redmine version 3.1.1.stable
Ruby version 1.9.3-p484 (2013-11-22) [x86_64-linux]
Rails version 4.2.4
Environment production
Database adapter Mysql2
SCM:
Subversion 1.8.8
Git
Filesystem
Redmine plugins:
no plugin installed