While there are a lot of pre-existing examples (like .nomedia) of this where the user doesn’t get to choose the magic file name, I do like your idea of joining “magic name” with “action”. I’d suggest having some defaulted suggstions such as the
The only thing that worries me is this opens up a lot of potential for somebody choosing something like
Thumbs.db as their magic file name and then complaining because Duplicati didn’t back up stuff they expected it to back up.
That makes me think the perfect place for this is actually in the Advanced Filters area where peopple are already able to do scary things.
And on top of that, there should be a “show me what folders or files will be EXCLUDED” feature for verification purposes. (Note that I only put that in because chances are that either already exists or was suggested by somebody 30 years ago. )