Multiple files upload in a custom Drupal form programmatically | Native, clean example Drupal code | Also see https://www.lucius.digital/en/blog/multiple-files-upload-custom-drupal-form-programmatically-native-clean-example-drupal-code

MessageForm.php

<?php

namespace Drupal\ol_messages\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Component\Utility\Html;
use Drupal\Component\Utility\Xss;
use Drupal\file\Entity\File;
use Drupal\ol_file\Entity\OlFile;
use Drupal\ol_main\Services\OlFiles;
use Drupal\ol_members\Services\OlMembers;
use Drupal\ol_messages\Services\OlMessages;
use Symfony\Component\DependencyInjection\ContainerInterface;


/**
 * Class MessageForm.
 */
class MessageForm extends FormBase {

  /**
   * @var $messages
   */
  protected $messages;

  /**
   * @var $files
   */
  protected $files;

  /**
   * @var $members
   */
  protected $members;

  /**
   * Class constructor.
   */
  public function __construct(OlMessages $messages, OlFiles $files, OlMembers $members) {
    $this->messages = $messages;
    $this->files = $files;
    $this->members = $members;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('olmessages.messages'),
      $container->get('olmain.files'),
      $container->get('olmembers.members')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'message_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, $op = null, $id = null) {

    // Defaults.
    $name = '';
    $body = '';
    $button_text = t('Submit');
    $hdd_file_location = $this->buildFileLocaton('message');

    // Handle edit vars.
    if ($op == 'edit'){
      $message_data = $this->getMessageData($id);
      $name = $message_data->name;
      $body = $message_data->body;
      $button_text = t('Update Message');
    }

    // Build form.
    $form['message_id'] = [
     '#type' => 'hidden',
     '#default_value' => $id,
     '#weight' => '0',
    ];
    $form['name'] = [
      '#type' => 'textfield',
      '#weight' => '10',
      '#default_value' => $name,
      '#required' => true,
      '#attributes' => array('placeholder' => t('Add a title...'), 'maxlength' => '150'),
    ];
    $form['body'] = [
      '#type' => 'text_format',
      '#format' => 'ol_rich_text',
      '#weight' => '20',
      '#default_value' => $body,
      '#required' => true,
    ];
    $form['files'] = array(
      '#title' => t('Attach files'),
      '#type' => 'managed_file',
      '#required' => FALSE,
      '#upload_location' => 'private://'.$hdd_file_location,
      '#multiple' => TRUE,
      '#upload_validators' => array(
        'file_validate_extensions' => $this->getAllowedFileExtensions(),
      ),
      '#weight' => '30',
    );
    $form['submit'] = [
      '#type' => 'submit',
      '#weight' => '100',
      '#attributes' => array('class' => array('btn btn-success')),
      '#value' => $button_text,
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {

    if (strlen($form_state->getValue('name')) > 128) {
      // Set an error for the form element with a key of "name".
      $form_state->setErrorByName('name', $this->t('Message not saved yet: title can\'t be more than 128 characters.'));
    }
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {

    // Get data.
    $id = Html::escape($form_state->getValue('message_id'));
    $name = Xss::filter($form_state->getValue('name'));
    $body = $form_state->getValue('body')['value'];
    $body = check_markup($body,'ol_rich_text');
    $files = $form_state->getValue('files');
    // Existing, update message.
    if(is_numeric($id)){
      $this->messages->updateMessage($id, $name, $body);
    }
    // New, save message.
    elseif(empty($id)){
      $id = $this->messages->saveMessage($name, $body);
    }
    if(!empty($files)) {
      $this->saveFiles($files, $id);
    }
  }

  /**
   * @param $files
   * @param $entity_id
   * @throws \Drupal\Core\Entity\EntityStorageException
   */
  private function saveFiles($files, $entity_id){
    // Needed for end-user message.
    $filenames = array();
    // Loop through uploaded files.
    foreach ($files as $fid) {
      $file = File::load($fid);
      $file->setPermanent();
      $file->save();
      $name = $file->getFilename();
      $filenames [] = $name;
      // Create ol_file entity
      $ol_file = OlFile::create([
        'name' => $name,
        'file_id' => $fid,
        'entity_id' => $entity_id,
      ]);
      $ol_file->save();
    }
    // Build message for end-user.
    // Count uploaded files.
    $files_count = count($filenames);
    // If there is 1 file uploaded.
    $filename =  ($files_count == 1) ? $filenames[0] : '';
    // If there are multiple files uploaded
    $filenames_imploded = ($files_count > 1) ? implode(', ', $filenames) : '';
    // Build message, based on file count.
    if ($filename){
      $message = t('One file uploaded successfully: ') .$filename;
    } elseif ($filenames_imploded) {
      $message = t('@files_count files uploaded successfully: ',
          array('@files_count' => $files_count)) .$filenames_imploded;
    }
    // Add message.
    \Drupal::messenger()->addMessage($message);
  }


  /**
   * @param $id
   * @return mixed
   */
  private function getMessageData($id){
    $query = \Drupal::database()->select('ol_message', 'mess');
    $query->addField('mess', 'body');
    $query->addField('mess', 'name');
    $query->condition('mess.id', $id);
    return $query->execute()->fetchObject();
  }

  /**
   * @param $entity_type
   * @return string
   */
  public function buildFileLocaton($entity_type){
    // Build file location
    return $entity_type.'/'.date('Y_W');
  }

  /**
   * @return array
   */
  private function getAllowedFileExtensions(){
    return array('jpg jpeg gif png txt doc docx zip xls xlsx pdf ppt pps odt ods odp');
  }
}