I’ve seen a few things telling of how to clean out the teeth of a file. One old-timey way, the source of which I cannot find was to hammer the end of a large nail flat and run that over the file groove-wise. The teeth of the file will cut the nail into a sort of rake which will force out any aluminum or other soft metal bits, even sawdust!
This video says pretty much the same thing:
So I don’t yet own a file card brush, but I had a few old keys to a padlock that broke. I ground the head of the key to a wide flat spot and mounted it in a small piece of scrap wood as a handle. The key is held in by friction.
After some use the key gained the grooves that clean out the teeth of the file you are working on. Keys are usually made of brass, so its a good use for old ones.
-- Ni faru ion el ligno!