.bts is BitTorrent Sync unfinished download
Key Features: .BTS File Extension
What is an .BTS file?
File extension bts is nowadays mostly related to BitTorrent Sync, a file synchronization tool based on P2P technology.
A .bts basically represents file download in progress with and is generated for the purposes of file allocation, to reserve the space for the file. When the download/sync is finished, the extension is changed back to the original.