The “.streams” folder is used by the SAMBA service to store metadata created by an NTFS file system feature called NTFS Alternate Data Streams (ADS). When a file that includes ADS metadata is transferred to a NAS shared folder using the protocol CIFS/SMB, its ADS metadata is stored as a file in the shared folder’s ".streams" folder.
https://www.qnap.com/ja-jp/how-to/faq/article/what-is-the-streams-folder-in-a-shared-folder
