Change Plone workflow
You should try this:

Go to portal_workflow -> contents
Copy & Paste simple_publication_workflow
Rename copy_of_simple_publication_workflow to something else (eg. my_files_workflow)
Go to my_files_workflow -> States -> published -> Permissions
Uncheck the unwanted Permissions

Now you can assign this new workflow to your files.
By default Files haven't workflow
So i put them the simple publication workflow (private -> submit -> publish)
I want the authenticated users could not modify the file when it is published
(on Plone 4.0.7)