What you're describing is just Finder caring. Linux doesn't care at all about your file extensions but Nautilus sure does.
In GNOME for example gio handles opening files in the "correct" application by way of the MIME database in /usr/share/application/mimeapps.list and ~/.local/share/applications/mimeapps.list.
LaunchServices definitely cares about extensions. The Mac OS has a strange history with extensions (classic Macs didn't really support them, but they were still a common convention), but file extensions are more than a browser-level construct.
