How can I create a filetype association that uses the filename's extension?

[Sietse Brouwer <sbbrouwer gmail com>]

Dear List,

I have several kinds of files that Gnome detects as "plain text
documents": some with the extension .py, some with .txt, some with
.mht . . . you get the picture. The .py files I'd like to open with
gVim, the .txt files in SciTE, the .mht files with Opera, et cetera.
However, when I try to change the "Open WIth" of any of these file
types [1], it changes for all plain text files. The behaviour I desire
is that I can associate different programs to files with different
extension.

Is it possible to persuade Gnome to treat files with the same MIME
type but different filename extensions separately?
If yes, how?
If not: do you think a feature request to add this functionality would
stand a chance, or has this debate already been had? (I couldn't find
any mentions in the archives, or elsewhere, apart from a few forlorn
forum posts.)

Kind regards, and a happy new year to you all,

Sietse
(Sietse Brouwer)

[1] via right-click > Open With > Other Application... > check
"Remember this application for 'plain text document";
or, alternatively, via right-click > Properties > Open With > pick one