Hey Sharkie, thx for the fast reply and i wish all the best to you to get better
I googled around a lil bit and i found this great little utility:
WMKits
How to set file association in windows mobile phone
The easiest way to change extension association on WM
You may want to consider to add it to your tool pack, is very useful
I fixed the little mistake and I can confirm that the ext .WMV was linked to HTCgallery (or something similar, can t recall).
Now video are starting perfectly in media player.
.MP4 issue not solved yet.