When I open an attached file in Org Mode with C-a o or its directory with C-a f it always opens in Emacs instead of the right external program or file browser.
[ What do I mean with "attached": I attach files to an outline with C-a c (or y or m or...). ]
What can I do about that? Problem ist solved.
The Problem appeared in the combination of Ubuntu 17.10 and Emacs 25.2 (or whatever is shipped with Ubuntu 17.10, definitel > 25).
With Windows 10 / Emacs 25.3 and Ubuntu 16.04.3 / emacs 24 shortcuts do what they should do.