file_managed_file_submit

file_managed_file_submit($form, FormStateInterface $form_state)

function file_managed_file_submit($form, FormStateInterface $form_state) {
  // Determine whether it was the upload or the remove button that was clicked,
  // and set $element to the managed_file element that contains that button.
  $parents = $form_state->getTriggeringElement()['#array_parents'];
  $button_key = array_pop($parents);
  $element = NestedArray::getValue($form, $parents);

  // No action is needed here for the upload button, because all file uploads on
  // the form are processed by \Drupal\file\Element\ManagedFile::valueCallback()
  // regardless of which button was clicked. Action is needed here for the
  // remove button, because we only remove a file in response to its remove
  // button being clicked.
  if ($button_key == 'remove_button') {
    $fids = array_keys($element['#files']);
    // Get files that will be removed.
    if ($element['#multiple']) {
      $remove_fids = array();
      foreach (Element::children($element) as $name) {
        if (strpos($name, 'file_') === 0 && $element[$name]['selected']['#value']) {
          $remove_fids[] = (int) substr($name, 5);
        }
      }
      $fids = array_diff($fids, $remove_fids);
    }
    else {
      // If we deal with single upload element remove the file and set
      // element's value to empty array (file could not be removed from
      // element if we don't do that).
      $remove_fids = $fids;
      $fids = array();
    }

    foreach ($remove_fids as $fid) {
      // If it's a temporary file we can safely remove it immediately, otherwise
      // it's up to the implementing module to remove usages of files to have them
      // removed.
      if ($element['#files'][$fid] && $element['#files'][$fid]->isTemporary()) {
        $element['#files'][$fid]->delete();
      }
    }
    // Update both $form_state->getValues() and FormState::$input to reflect
    // that the file has been removed, so that the form is rebuilt correctly.
    // $form_state->getValues() must be updated in case additional submit
    // handlers run, and for form building functions that run during the
    // rebuild, such as when the managed_file element is part of a field widget.
    // FormState::$input must be updated so that
    // \Drupal\file\Element\ManagedFile::valueCallback() has correct information
    // during the rebuild.
    $form_state->setValueForElement($element['fids'], implode(' ', $fids));
    NestedArray::setValue($form_state->getUserInput(), $element['fids']['#parents'], implode(' ', $fids));
  }

  // Set the form to rebuild so that $form is correctly updated in response to
  // processing the file removal. Since this function did not change $form_state
  // if the upload button was clicked, a rebuild isn't necessary in that
  // situation and calling $form_state->disableRedirect() would suffice.
  // However, we choose to always rebuild, to keep the form processing workflow
  // consistent between the two buttons.
  $form_state->setRebuild();
}