There are definitely a number of programs out there for Windows that watch folders for you and can then run arbitrary commands/scripts on the files that appear - after they've been written. Such as DirMon

When a file is being written it'll be locked by the program doing the writing, will it not? When that lock has been released, it's ready to go. In any case, plenty of software supports this kind of file monitoring feature, so it's probably something pretty straight forward on the OSes that they're written for.