Uploading xlsx-file changes permissions of the file? Errors when loading via PHPExcel

Question

My goal is to let the user upload a xlsx-file and then read the file to create objects. So i created a file upload page in my custom module where the user can upload a file via the 'file' form field and the read the file via PHPExcel (which was already present when i started working on the module so i used this instead of PHPSpreadsheet).

The problem is, that after uploading the file and loading it via PHPExcel, i get several errors:

ZipArchive::getFromName(): Invalid or uninitialized Zip object in PHPExcel_Reader_Excel2007->_getFromZipArchive()

ZipArchive::close(): Invalid or uninitialized Zip object in PHPExcel_Reader_Excel2007->load()

Trying to get property 'Relationship' of non-object in PHPExcel_Reader_Excel2007->load()

When reading the file directly from a folder via PHPExcel it works (with the same file) and the uploaded xlsx file opens without problems in Excel 2016.

After reseraching i suspect that this has something to do with file permissions and the uploaded file indeeddiffers from the original file in the permissions:

In the security tab of the uploaded file an entry "Authentificated Users" was added.

My question is: What causes my problems? Am i on the right ttrack? Why does this permission entry gets added? How can i stop the uploader to do this?

My code:

$form['file'] = [
    '#type' => 'file',
    '#title' => t('Choose a xlsx-file based on the template.'),
  ];

and:

  // Process the file provided by the user.
  $form_file = $form_state['values']['file'];

  $file_destination = 'public://';

  $validators = ['file_validate_extensions' => ['xlsx']];

  $file = file_save_upload('file', $validators, $file_destination, FILE_EXISTS_REPLACE);

  if (!$file) {
    watchdog('my_module', 'PHPExcel could not import! Import file missing.');
    return NULL;
  }

  $file_name = $file->filename;
  $import_file = $file_destination.$file_name;

  // Load the library and initialize.
  module_load_include('php', 'my_module', 'lib/phpexcel_1.8.0/PHPExcel');

  // Load the import file.
  try {
    $objReader = PHPExcel_IOFactory::createReader('Excel2007');
    $objPHPExcel = $objReader->load($import_file);
  }

  catch (Exception $exception) {
    drupal_set_message($exception);
    watchdog('rdp_cellmodel', $exception);
  }

  // read file etc.