add.ctp

<nav class="large-3 medium-4 columns" id="actions-sidebar">
    <ul class="side-nav">
        <li class="heading"><?= __('Actions') ?></li>
        <li><?= $this->Html->link(__('List Users'), ['action' => 'index']) ?></li>
        <li><?= $this->Html->link(__('List Bookmarks'), ['controller' => 'Bookmarks', 'action' => 'index']) ?></li>
        <li><?= $this->Html->link(__('New Bookmark'), ['controller' => 'Bookmarks', 'action' => 'add']) ?></li>
    </ul>
</nav>
<div class="users form large-9 medium-8 columns content">
    <?= $this->Form->create($user, ['type' => 'file']) ?>
    <fieldset>
        <legend><?= __('Add User') ?></legend>
        <?php
            echo $this->Form->input('email');
            echo $this->Form->input('password');
            echo $this->Form->input('photo[]', ['type' => 'file', 'multiple' => 'true', 'label' => 'Add Some Photos']);
            echo $this->Form->input('photo_dir', array('type' => 'hidden'));
        ?>
    </fieldset>
    <?= $this->Form->button(__('Submit')) ?>
    <?= $this->Form->end() ?>
</div>

UsersController.php

  public function add()
    {
        $user = $this->Users->newEntity();
        if ($this->request->is('post')) {
            $user = $this->Users->patchEntity($user, $this->request->data);
            if ($this->Users->save($user)) {
                $this->Flash->success(__('The user has been saved.'));
                return $this->redirect(['action' => 'index']);
            } else {
                $this->Flash->error(__('The user could not be saved. Please, try again.'));
            }
        }
        $this->set(compact('user'));
        $this->set('_serialize', ['user']);
    }

UsersTable.php

<?php
namespace App\Model\Table;

use App\Model\Entity\User;
use Cake\ORM\Query;
use Cake\ORM\RulesChecker;
use Cake\ORM\Table;
use Cake\Validation\Validator;

/**
 * Users Model
 *
 * @property \Cake\ORM\Association\HasMany $Bookmarks
 */
class UsersTable extends Table
{

    /**
     * Initialize method
     *
     * @param array $config The configuration for the Table.
     * @return void
     */
    public function initialize(array $config)
    {
        parent::initialize($config);

        $this->table('users');
        $this->displayField('id');
        $this->primaryKey('id');

        $this->addBehavior('Josegonzalez/Upload.Upload', [
            'photo'
        ]);


        $this->addBehavior('Timestamp');

        $this->hasMany('Bookmarks', [
            'foreignKey' => 'user_id'
        ]);
    }

    /**
     * Default validation rules.
     *
     * @param \Cake\Validation\Validator $validator Validator instance.
     * @return \Cake\Validation\Validator
     */
    public function validationDefault(Validator $validator)
    {
        $validator
            ->integer('id')
            ->allowEmpty('id', 'create');

        $validator
            ->email('email')
            ->requirePresence('email', 'create')
            ->notEmpty('email');

        $validator
            ->requirePresence('password', 'create')
            ->notEmpty('password');

        return $validator;
    }

    /**
     * Returns a rules checker object that will be used for validating
     * application integrity.
     *
     * @param \Cake\ORM\RulesChecker $rules The rules object to be modified.
     * @return \Cake\ORM\RulesChecker
     */
    public function buildRules(RulesChecker $rules)
    {
        $rules->add($rules->isUnique(['email']));
        return $rules;
    }
}