private void SaveImage(FsPath file, FsPath targetdir, ILog log, SKData data, string?extensionOverride)
        {
            FsPath target = targetdir.Combine(file.Filename);

            if (extensionOverride != null)
            {
                var newname = Path.ChangeExtension(file.Filename, extensionOverride);
                target = targetdir.Combine(newname);
            }
            using (var stream = target.CreateStream(log))
            {
                log.Detail("Saving image: {0}", target);
                data.SaveTo(stream);
            }
        }