OK, had a chance to test more. Here are my findings with Version v20210302.0.82937 built on Tuesday, March 2, 2021 4:36:51 PM fv-az15-142 using the “.kopiaignore” file.
I can get it to exclude everything except certain file types if I also explicitly include each directory that makes up the path to the files I want included. For example, if I want .ai files in the path /a/b/c/d I need to do:
*
!a/
!a/b/
!a/b/c/
!a/b/c/d/
!*.ai
However, I want it to match .ai files for all 100’s of subfolders inside of /a/ so this won’t do.
After re-reading the .gitignore documentation, I found the command, ** which should allow me to not only list the directory in the above example with 1 line (!a/**/d) but it should allow me to get exactly what I want with:
*
!a/**
!*.ai
…but it doesn’t work. I tried using ** every which way and couldn’t get it to do a thing.
This has been proven false. The tools at one’s disposal to do such a thing exist, and are semi-documented via .gitignore, but they are a bit hostile.
Also, I fully reserve the right to claim user error 
. Most of my findings and frustrations so far are the result of guess and check - I’m just poking at this thing with command after command and seeing how it responds and, like a slot machine, I can’t seem to stop until I’ve hit the jackpot.
Ideas?