Adding function for visiting unused files

I often need this to now if I can delete some unused stuff without losing anything of interest. 
I'm not concerned bye the fact that my binding might not be the best one. What do you think?
Given the Dired binding, I think `!` makes sense.  The other one the comes to mind is `M-RET`.  I even wonder if the external command should be on RET because annexed files are likey in formats that the user won't want to try to open in Emacs (at least this is true for how I use git annex).