Just wanna add that I had trouble getting this to work aswell. But I’m using Sage theme from Roots, and you need to hook into the filters like this:
add_filter('upload_dir', __NAMESPACE__ . '\\my_upload_directory');
add_filter('acf/upload_prefilter/name=preview_video', __NAMESPACE__ . '\\my_acf_upload_prefilter');
Otherwise it won’t be able to locate the filters.
