private void RemoveButton_Click(object sender, EventArgs e)
        {
            if (FileListBox.Items.Count > 0)
            {
                var newList = new List <string>();

                for (var index = 0; index < FileListBox.Items.Count; index++)
                {
                    if (!FileListBox.GetItemChecked(index))
                    {
                        newList.Add(FileListBox.Items[index].ToString());
                    }
                }

                FileListBox.Items.Clear();

                foreach (var item in newList)
                {
                    FileListBox.Items.Add(item);
                }

                if (FileListBox.Items.Count > 0)
                {
                    SelectMode = !SelectMode;
                }
                else
                {
                    SelectMode = true;
                }
            }
        }