I used my number 4 file to do some work on 22kt. yellow gold, now
my file is clogged with the gold.
Try an ultrasonic cleaner, followed by, if you have it, a steam
cleaner. Usually one or the other will do the trick.
You can use the very sharp point of a sewing needle or something,
but that's tedious in the extreme, as you have to run it down each
and every tooth. And with the finest grade files, even that gets
tricky. Sometimes you can take a piece of soft brass or copper, and
"strike" it parallel with the teeth, so that the teeth press grooves
into the edge of the soft metal. You're going with the teeth, so the
brass or copper isn't cut, but being pressed into the shape of the
teeth, it can get down into them a bit. You do this usually with
something like the corner of a piece of heavy sheet metal, or
something like that, so the copper or brass piece is an edge (so it
gets cut into) that extends across the width of several of the file
teeth. Not perfect, and works better with coarser files, but try it.
Might do it for you. Sorry for the probably garbled explanation...