@Test
public void testValidateNonEmptyFile() {

    final ValidatorWithReasonCallback validatorWithReasonCallback = mock(ValidatorWithReasonCallback.class);
    when(uploadWidget.getFilenameSelectedToUpload()).thenReturn("table.xls");
    handler.validate("filename", validatorWithReasonCallback);
    verify(uploadWidget, never()).addStyleName(ValidationState.ERROR.getCssName());
}