Change the .fbx file association to blender_to_os.bat. To do this, right-click any .fbx file, click Properties, and change the Opens with preference. Click on more apps↓, scroll down and select Look ...

Place your code in separate files. Best pratice is to put operators, menus, views etc. everything that needs a register and unregister function into its own file and let 'init.py' initialize the ...