Neos\Neos\Controller\Backend\ContentController::initializeUploadAssetAction PHP Method

    public function initializeUploadAssetAction()
    {
        $propertyMappingConfiguration = $this->arguments->getArgument('asset')->getPropertyMappingConfiguration();
        $propertyMappingConfiguration->allowAllProperties();
        $propertyMappingConfiguration->setTypeConverterOption(PersistentObjectConverter::class, PersistentObjectConverter::CONFIGURATION_CREATION_ALLOWED, true);
        $propertyMappingConfiguration->setTypeConverterOption(AssetInterfaceConverter::class, AssetInterfaceConverter::CONFIGURATION_ONE_PER_RESOURCE, true);
        $propertyMappingConfiguration->allowCreationForSubProperty('resource');
    }