PhpSpreadsheet fails to identify my file in public://

I'm working on a module to import a Excel file and using PhpSpreadsheet class. Now, if my uploaded file is out of public://, I have no problems at all importing it. But if I have it in public://, I get PHP message: Uncaught PHP Exception InvalidArgumentException: "File "" does not exist." at File.php line 137. Line 137 throws an exception if the file is not a file, but dpm says it is. What am I doing wrong? Below is my code:

public function submitForm(array &$form, FormStateInterface $form_state) {
        $connection = \Drupal::database();
        $fid = $form_state->getValue("xlsx_upload");
        $input = File::load(reset($fid));
        $input->setPermanent();
        $uri = \Drupal::service('file_system')->realpath($input->getFileUri());
        //$uri = '/home/joe/file.xslx'; <--- this works perfectly.
        $reader = \PhpOffice\PhpSpreadsheet\IOFactory::createReaderForFile($uri);
        $spreadsheet = $reader->load($filename);
        $dataArray = $spreadsheet->getActiveSheet()->toArray();
        $connection->truncate("mitacan")->execute();
        $connection->query("SET NAMES utf8");
        foreach ($dataArray as $dataItem) {
          $connection->insert('mitacan')
          ->fields([
            'stokkodu'		=> $dataItem[0],
            'logo' 			=> $dataItem[1],
            'resim1' 		=> $dataItem[2],
            'grupkodu' 		=> $dataItem[3],
            'grupadi' 		=> $dataItem[4],
            'grupindex' 	=> $dataItem[5],
            'paketadedi'	=> $dataItem[6],
          ])
          ->execute();
        }
        drupal_set_message('Excel verisi veritabanına eklendi.');
      }

Thank you.