Finally, I found a free SFTP server for my file server.
http://www.k2sxs.com/silvershield/
Although it limited only one client and max 3 session on free version. However, it run as service nature and natively support file name encoding UTF8 which is enough for me.
GoodReader syncing through 3G
SilverSHielD Management Console screen
Security Settings – anti hammer support
Expert Settings – just show off how many type of SFTP / encryption method supported
Add/Edit User screen. Support Windows/AD account, command shell login
