These days, there are mature programs for launching an arbitrary file with
an appropriate viewer: run-mailcap (which is mature to the point of
obsolescence) and xdg-open on GNU systems; open on Mac OS X; start on MS
Windows).

xdg-open already works reasonably well:
http://lists.gnu.org/archive/html/help-gnu-emacs/2012-01/msg00143.html

(I have a fix for the "multiple files" problem, which I'll be sending to
xdg-utils upstream when I can.)

Hence it would seem sensible to have a new default for
dired-guess-shell-alist-default, which matches any file and returns the
appropriate command for the OS, probably doing some testing to see which
commands are available on a GNU system ("xdg-open", "gvfs-open", "see"…).

The user then only needs to understand and configure one command-launching
system, and the coverage of different filetypes is much more comprehensive
in most such systems than in dired-x.el.

An advantage to Emacs is that this system could then be used elsewhere. For
example, org-mode already has a similar, separate system, org-file-apps,
which might benefit from building on dired-x, and only retaining its own
configuration where it's desirable to open a file "internally" to org-mode.

-- 
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