namespace App\Admin;

use SilverStripe\Admin\ModelAdmin;
use ilateral\SilverStripe\SlightlyBetterBulkLoader\CsvBulkLoader;

class MyModelAdmin extends ModelAdmin
{
    private static $managed_models = [
        MyDataObject::class
    ];

    private static $model_importers = [
        MyDataObject::class => CsvBulkLoader::class
    ];
}

use SilverStripe\Forms\Form;
use ilateral\SilverStripe\SlightlyBetterBulkLoader\CsvBulkLoader;

class MyImportForm extends Form
{
    public function import($data, $form)
    {
        $loader = CsvBulkLoader::create();
        $results = $loader->load($_FILES['_CsvFile']['tmp_name']);

        $form->sessionMessage(
            $results->getMessagesString("</br>"),
            $results->getMessageType(),
            ValidationResult::CAST_HTML
        );

        return $this->redirectBack();
    }
}