How to open some files based on extension rather than mime type?

[John Hupp]

I don't understand mime types very well, but it seems like the net 
effect as implemented is that they don't result in very granular control.

To be specific, I find that if I use LibreOffice Writer to create an 
.odt file, a double-click on the file in pcmanfm results in AbiWord 
opening the file.

I would like LibreOffice to open odt files, AbiWord to open abw files, 
and leafpad to open plain text files.  But lxsession-default-apps merely 
sets a "Document" launcher.

Can set up finer control to open some files according to their extension?