Unable to validate the uploaded file

I am trying to upload only the files with txt format but validation fails everytime.

protected function validator(array $data, $id = null) { $rules = array(); $rules['my_file'] = 'required|mimes:text/plain'; if(!is_null($id)) { $rules['file_title'] = 'required|unique:uploaded_classes,file_title,'.$id; }

return Validator::make($data, $rules);

}