trying to understand XDG - a plea for help

[Tomasz Chmielewski]

Hello,

As recently my primary distro began to switch to XDG-style menu, and 
generally extension management (like *.pdf, *.avi, *.mp3), I noticed I 
can't do things I could do easily in the past.

Namely: I can use *only* the extensions provided by the distribution, 
and to programs provided by the distribution.

And therefore:

Why can't I make my own *.tomek extension and attach
it to /usr/local/bin/tomek program (written by me)?

Why can't I write my own *.mp3 player and set it to mp3 files by default?

If a user downloads Adobe Acrobat Reader, why he/she can't then 
associate it with pdf extension?

If a user downloads Real Player, why he/she can't associate it with Real 
Player files?



So if anyone could tell me, if it's the distribution which broke this 
XDG concept, or maybe it's the XDG concept to disallow a typical Joe 
user to attach file extensions to his own programs, it would make my day.


Tomek