public class FileEditor : UITypeEditor {
    private OpenFileDialog openFileDialog;
    public override object EditValue(ITypeDescriptorContext context,  IServiceProvider  provider, object value) {
        if (openFileDialog == null){
            openFileDialog = new OpenFileDialog();
        }
        openFileDialog.DefaultExt = "dat";
        openFileDialog.Multiselect = false;
        openFileDialog.Title = "Select Attachment";
        openFileDialog.CheckFileExists = true;
        openFileDialog.CheckPathExists = true;
        if (openFileDialog.ShowDialog() == DialogResult.OK)
            return new FileName(openFileDialog.FileName);
        return null;
    }
    public override UITypeEditorEditStyle GetEditStyle(ITypeDescriptorContext context) {
        return UITypeEditorEditStyle.Modal;
    }
}