The files tab is certainly a great feature saving developers from needing to user a file manager for browsing/opening files outside of Geany. Could it be possible to add a feature to "Make a new directory" and to change the chmod permissions on the file using the files tab itself. Speaking personally this would save me a tone of time developing and fine tuning my web applications.
The filebrowser plugin shipped with Geany is as small as it is on purpose. We think the file browser should stay a simple file browser and not a file manager.
Anyway, the treebrowser plugin from the Geany-Plugins project has many enhancements and also your request already implemented. Maybe that helps you.
In the treebrowser plugin are implemented some basic filesytem functionalities, you can take a view at it http://plugins.geany.org/treebrowser.html
About the chmod permissions, I am not sure why it have to be implemented, but if you give me a reason, I will try to implement such option.