> how does org-mode 'decide' whether a linked filed (C-c C-l) should be opened (C-c C-o) inside an Emacs buffer or opened by an external
> program? I am using Emacs 24.3 for Windows and org-mode 8.2

 
reading the /HOME/USER/.mailcap file.

please, have a look at the  FAQ:
http://orgmode.org/worg/org-faq.html#text-external-application-launched-to-open-file-link