SftpGo

Fully featured and highly configurable SFTP server

Homepage : https://sftpgo.com/

Applicable models: x.64 series
Applicable TOS4: 
Applicable TOS5: 5.1.12 or later versions

App WebUI for additional settings can be accesses at:

TOS4: http(s)://yournasip:port/modules//www
TOS5: click on Support & Help from app description inside the App Center

About:

Fully featured and highly configurable SFTP server with optional HTTP/S, FTP/S and WebDAV support. Several storage backends are supported: local filesystem, encrypted local filesystem, S3 (compatible) Object Storage, Google Cloud Storage, Azure Blob Storage, SFTP.

Features
Support for serving local filesystem, encrypted local filesystem, S3 Compatible Object Storage, Google Cloud Storage, Azure Blob Storage or other SFTP accounts over SFTP/SCP/FTP/WebDAV.
Virtual folders are supported: a virtual folder can use any of the supported storage backends. So you can have, for example, a user with the S3 backend mapping a GCS bucket (or part of it) on a specified path and an encrypted local filesystem on another one. Virtual folders can be private or shared among multiple users, for shared virtual folders you can define different quota limits for each user.
Configurable custom commands and/or HTTP hooks on upload, pre-upload, download, pre-download, delete, pre-delete, rename, mkdir, rmdir on SSH commands and on user add, update and delete.
Virtual accounts stored within a "data provider".
SQLite, MySQL, PostgreSQL, CockroachDB, Bolt (key/value store in pure Go) and in-memory data providers are supported.
and more…