Hi Julien,
Are you serious here.
Lets see what do you need to install wp-filemanager
1) Administrative access to wordpress
2) write access to file system by web server
Frankly with both these access if an attacker is installing wp-filemanager to edit files i feel pity on the attacking skills of the person. If you have both above listed access you would mostly also have edit theme & plugins files access and should be able to edit theme directly and put exploit code.
If you were a victim please ensure your username and passwords are complex enough to be be easy to guess.
