Trait for non versioned SilverStripe 4 DataObjects. Files associated to DataObjects that are non-versioned do not get published by default.

PublishRelatedFilesTrait.php

<?php

namespace MyProject\Traits;

use SilverStripe\Assets\Image;

/**
 * Class PublishRelatedFilesTrait
 * @package MyProject\Traits
 */
trait PublishRelatedFilesTrait
{
    /**
     * Publish our dependent objects.
     */
    public function onAfterWrite()
    {

        $hasOnes = $this->stat('has_one');
        foreach ($hasOnes as $relation => $class) {
            if ($class == Image::class || $class == File::class) {
                $this->publishRelatedObject($this->$relation());
            }
        }

        $hasMultiples = array_merge($this->stat('has_many'), $this->stat('many_many'));
        foreach ($hasMultiples as $relation => $class) {
            if ($class == Image::class || $class == File::class) {
                foreach ($this->$relation() as $object) {
                    $this->publishRelatedObject($object);
                }
            }
        }

        parent::onAfterWrite();
    }

    /**
     * @param $object
     */
    protected function publishRelatedObject($object)
    {
        if ($object && $object->exists()) {
            $object->publishSingle();
        }
    }
}