fix: improve check if external storage backend is local

Signed-off-by: Robin Appelman <robin@icewind.nl>

# Conflicts:
#	apps/files_external/lib/Controller/GlobalStoragesController.php
#	apps/files_external/lib/Controller/StoragesController.php

Author: icewind1991

apps/files_external/lib/Controller/StoragesController.php

@@ -9,6 +9,7 @@
 
 use OCA\Files_External\Lib\Auth\AuthMechanism;
 use OCA\Files_External\Lib\Backend\Backend;
+use OCA\Files_External\Lib\Backend\Local;
 use OCA\Files_External\Lib\InsufficientDataForMeaningfulAnswerException;
 use OCA\Files_External\Lib\StorageConfig;
 use OCA\Files_External\NotFoundException;
@@ -78,7 +79,7 @@ protected function createStorage(
 		?int $priority = null,
 	) {
 		$canCreateNewLocalStorage = $this->config->getSystemValue('files_external_allow_create_new_local', true);
-		if (!$canCreateNewLocalStorage && $backend === 'local') {
+		if (!$canCreateNewLocalStorage && $this->backendService->getBackend($backend) instanceof Local) {
 			return new DataResponse(
 				[
 					'message' => $this->l10n->t('Forbidden to manage local mounts')