// getting the input element
  $fileInput = $driver->findElement(WebDriverBy::id('file_input'));
  // set the file detector on the file_input element
  $fileInput->setFileDetector(new LocalFileDetector());
  // upload the file and immediatelly submit the form
  $fileInput->sendKeys($filePath)
      ->submit();
  // you can also just fill the file input and submit the form later
  $fileInput->sendKeys($filePath);
  // ... do some other stuff in the form, then submit the whole form:
  $form->submit(); // if form is WebDriverElement representing <form>