I do not use LXDE as my default user interface, but I have seen it and its quite cool. There is no reason to switch from GNOME to LXDE. I am not trying to say that LXDE is not a good desktop environment, but GNOME has everything that a linux user requires.
I am not sure but it just uses the default applications which are set to the desktop. If you want to take a closer look then check this file
So, any one know about the config file or something like that so that I can set default applications to open various file formats.
Hope this information has helped you. Any more queries do post back.