fix: added file extension for dummy file name in project import (#561)

According to the workaround mentioned in https://gitlab.com/gitlab-org/gitlab-foss/issues/50944#note_101737263

Author: SirkoS

src/core/services/ProjectImportExport.ts

@@ -23,7 +23,7 @@ export class ProjectImportExport extends BaseService {
     const form = new FormData();
 
     const defaultMetadata: UploadMetadata = {
-      filename: Date.now().toString(),
+      filename: `${Date.now().toString()}.tar.gz`,
       contentType: 'application/octet-stream',
     };