bootstrap.php

<?php
Type::map('file', 'App\Database\Type\FileType');

FileType.php

<?php
namespace App\Database\Type;

use Cake\Database\Driver;
use Cake\Database\Type;

class FileType extends Type
{
    /**
     * Marshalls flat data into PHP objects.
     *
     * Most useful for converting request data into PHP objects
     * that make sense for the rest of the ORM/Database layers.
     *
     * @param mixed $value The value to convert.
     * @return mixed Converted value.
     */
    public function marshal($value)
    {
        return $value;
    }

    /**
     * {@inheritDoc}
     */
    public function toDatabase($value, Driver $driver)
    {
        return $value;
    }

    /**
     * {@inheritDoc}
     */
    public function toPHP($value, Driver $driver)
    {
        return $value;
    }
}

SomeController.php

<?php
namespace App\Controller;

use App\Controller\AppController;
use Cake\Event\Event;

class SomeController extends AppController
{
    function someMethod()
    {
        $id = 'some';
        $user = $this->Users->find()->where(['Users.id' => $id])
            ->contain([])->first();

        if ($this->request->is(['post', 'put'])) {
            $data = [
                // Here is our field with a custom type
                'pic' => $this->request->getData('pic'),
            ];

            // LOOK MA, NO LOGIC HERE. Everything has been moved to the Model
            $this->Users->patchEntity($user, $data);

            if ($this->Users->save($user)) {
                $this->Flash->success(__('Message.'));

                return $this->redirect('/your/url');
            }
        }
}

SomeTable.php

<?php
namespace App\Model\Table;

use Cake\ORM\Query;
use Cake\ORM\RulesChecker;
use Cake\ORM\Table;
use Cake\Validation\Validator;
use Cake\Database\Schema\TableSchema;
use App\Model\Validation\CommonValidator;

class UsersTable extends Table
{
    protected function _initializeSchema(TableSchema $schema)
    {
        $schema->setColumnType('pic', 'file');

        return $schema;
    }
    

    /**
     * Default validation rules.
     *
     * @param \Cake\Validation\Validator $validator Validator instance.
     * @return \Cake\Validation\Validator
     */
    public function validationDefault(Validator $validator)
    {
        // Your custom validator here
        $validator->setProvider('common', new CommonValidator());

        $validator->allowEmpty('pic');

        $validator->add('pic', 'valid-image', [
            'rule' => 'jpegPicture',
            // Apply a method from your custom validator
            'provider' => 'common',
            'message' => __('Only JPEG images are allowed')
        ]);

        return $validator;
    }


    public function beforeSave(Event $event, Entity $entity, ArrayObject $options)
    {
        if (!empty($entity->pic['tmp_name']) && is_uploaded_file($entity->pic['tmp_name'])) {
            try {
                // My custom uploader. Use your own logic
                $imager = new ImagerUtility();
                $name = $imager->generateMd5Filename($entity->pic['tmp_name']) . '.jpg';
                $imager->resizeAvatar($entity->pic['tmp_name'], $name);
                $entity->set('pic', $name);
            } catch (\Exception $e) {
                Log::debug($e->getMessage());
                $entity->set('pic', $entity->getOriginal('pic'));
            }
        } else {
            $entity->set('pic', $entity->getOriginal('pic'));
        }
    }
}