@zeddock, I am saying that I think the /usr/share/applications/defaults.txt is not the governing file, but may be changed during the boot process depending on the actual user login. I have seen this occur as blueplastic reported. What the GUI does is change not all users defaults, but seems to set up a preferences configuration file in the individual's /home/<username>/ folder someplace (again, I haven't investigated enough to know where) which overrides the global default.
I don't want to imply that a GUI is better than the CLI, but first you have to know exactly what file is the controlling configuration file. As an analogy, look at the GRUB 2 system.
There probably is a more 'elegant' solution, but I went with what I found to work which was simplest. I know, no cookie, and no geek credit.