First off, let me switch to talking about CreationKit.exe instead of NifSkope since it's the wider case and it's an official part of the game so supporting it's proper functionality is palatable. Now, I'm not talking about inspecting *.nif files from MO. I'm talking about the system-wide associations. So, in my case, say I'm working on a an armor piece and I'm constantly opening and reopening *.nif files from the file manager (My Computer/explorer.exe) since the nif plugins for 3dsmax and maya are buggy requiring frequent export/imports. Normally the association is to CreationKit.exe but if I'll just double click the file in the file manager following this direct association then I'll get a CreationKit.exe instance that's working outside MO virtual file system and it won't have access to my mods and overwrites where I actually store my modified textures and such. So, my suggestion is to modify MO to pipe (pass-on) the extra arguments it gets in stdin to it's stdout. Then write a small batch file like the one I showed above and associate *.nif files to that. Btw, my current solution is to open CreationKit.exe (well, actually I use NifSkope for these things but for the sake of this example) from MO and then open the file I want from there. It's a clunky and slow workflow and makes my life hard since I also need to restart Skyrim frequently to test out mesh adjustments if I'm fitting a piece of armor to a body.