As hinted below, stages are not the only way to add a file to the row. You can upload a file directly to a row. Perform a multipart post to the desired row's attachment key. e.g /lgc/service/file/upload/customeritem/62d257af86b992f81f41b25a8faf860e
If you re-invoke the List File Metas api for that row, you should now see the files added by attaching a stage and the files uploaded directly.