I dont think it is wise to change the File label . And if you do so in a future update of the crm your change will be overidden to the default value. I dont think its a supported change to the ribbon.
The only thing i can say is check out this link it may help you out with the ribbon customizations.
http://howto-mscrm.blogspot.com/2011/04/how-to-series-6-how-to-overrideenable.html