.BNS File Extension
File TypePortal Bonus Map Script
What is a BNS file?
Game file used by Portal, a first-person puzzle solving game; stores a plain text script that defines information about a Portal map (.BSP file); includes the map name, thumbnail image file references, comments about the map, and a list of challenges to complete.
BNS files are included as part of a .BMZ bonus map zip file, which can be installed to add bonus maps to Portal.