OroPlatform: add file relation to an entity

Context

I'm currently trying to add file relation to an entity of my OroPlatform project. The goal is to give the opportunity to the user to upload files on the create/update view of a specific entity.

I have read in the OroPlatform documentation that I have to create a new migration : https://doc.oroinc.com/master/backend/bundles/platform/AttachmentBundle/attachment-bundle-config/#attachment-bundle-file-types

Here is the migration I have created :

<?php

namespace Baltimore\Bundle\AppBundle\Migrations\Schema\v1_1_1;

use Doctrine\DBAL\Schema\Schema;

use Oro\Bundle\AttachmentBundle\Migration\Extension\AttachmentExtension;
use Oro\Bundle\AttachmentBundle\Migration\Extension\AttachmentExtensionAwareInterface;
use Oro\Bundle\MigrationBundle\Migration\Migration;
use Oro\Bundle\MigrationBundle\Migration\QueryBag;

class AddAttachmentToBusinessUnit implements Migration, AttachmentExtensionAwareInterface
{
    /** @var AttachmentExtension */
    protected $attachmentExtension;

    /**
     * {@inheritdoc}
     */
    public function setAttachmentExtension(AttachmentExtension $attachmentExtension)
    {
        $this->attachmentExtension = $attachmentExtension;
    }

    /**
     * {@inheritdoc}
     */
    public function up(Schema $schema, QueryBag $queries)
    {
        $this->attachmentExtension->addFileRelation(
            $schema,
            'oro_business_unit',
            'document',
            [],
            15
        );
    }
}

And I have added this line to the update.html.twig of my entity: form_row(form.document)

Issue

When I try to access the update page, I have the following error :

"Neither the property "document" nor one of the methods "document()", "getdocument()"/"isdocument()"/"hasdocument()" or "__call()" exist and have public access in class "Symfony\Component\Form\FormView"."

I have warmup the cache in order to generate the accessors for my class and the method getDocument() exist in the generated file EX_OroOrganizationBundle_BusinessUnit.php

Also, it seems that the property has been successfully added to my entity :

Finally my needs has changed, but I think I have the answer to solve this problem.
Here is what I will do:

/**
* {@inheritdoc}
*/
public function up(Schema $schema, QueryBag $queries)
{
   $this->attachmentExtension->addFileRelation(
      $schema,
      'oro_business_unit',
      'document',
      [
         'extend' => [
             'owner' => ExtendScope::OWNER_CUSTOM,
             'is_extend' => true
          ],
          'entity' => ['label'=>'oro.organization.businessunit.entity_label'],
      ],
      15
   );
}