I used to use an old Linux file manager called DFM back in 2002 to 2006 or so. Doesn't seem to be online any longer, but this looks like a copy of the original website sans graphics: http://distro.ibiblio.org/amigolinux/download/DeskTop/IconMg...

I have the source code saved somewhere.

This file manager had one nice feature that I haven't seen again: the use of a plain text config file to set which commands are used to open particular files. It might have used regex. You could also override the configuration manually by file too in the properties dialog box. If I recall correctly the icons were also set in the same file and also could be manually set.

The file manager is obsolete otherwise, but from time to time I have wanted to have the same feature again. You could easily get very precise this way. I use Thunar now, but maybe there's another file manager that can be configured more to my liking.