Thanks for the reply.
Yes, I have these buttons. Installing app-install-data allows the program to search on-line for application to install to deal with said file.
This is not what I want
I have the applicaion right here on the desktop that I want to associate with this file type, and Cinnamon wont let me add the executable of that file so I can open it easily.
As I say, I compile, lets say Blender, from source, becasue the repository versions are buggy and unsupported by blender.org anyway, and I want associate .blend files with the executable inside the /blender folder on my desktop, because that is the blender I will be using anyway for all my Blending. Can't do it. Which completely blows.
Cheers anyway. All the best.EDIT
I'll say how I think it should work as a BARE MINIMUM...
It should be that when you right click a file, and choose properties -> open with, or open with directly, it gives you the list of application that are available in the MENU - accessories, games, graphics, internet.. etc. Menu items can be easily added, their executable location set and all that, if associating a file directly without the menu is too difficult (whatever)..