public bool FilesFilter(object item)
        {
            if (item is FileViewModel fi)
            {
                var visible = ExtensionsFilters.Count == 0 || ExtensionsFilters.Contains(fi.Extension);
                visible &= string.IsNullOrEmpty(FilenameFilter) || fi.Name.ToLowerInvariant().Contains(FilenameFilter);
                return(visible);
            }

            return(false);
        }
        private void FileExtensionButton_Click(object sender, RoutedEventArgs e)
        {
            // hide others with unselected extension
            if (sender is ToggleButton tb && tb.Content is string ext)
            {
                if (tb.IsChecked == true)
                {
                    if (!ExtensionsFilters.Contains(ext))
                    {
                        ExtensionsFilters.Add(ext);
                    }
                }
                else
                {
                    ExtensionsFilters.Remove(ext);
                }

                FilesView.Refresh();
            }
        }