A BMZ file is a Bonus map file used by the 3D puzzle-solving game Portal developed by Valve. It contains a bonus map that can be created by third-party community developers as well and imported into the game using inbuilt UI. BMZ comes as ZIP archive file, having BSP, TGA and BNS files inside it. BMZ files can be opened using Valve Portal and Valve Portal 2.
BMZ File Format
BMZ are saved as ZIP files and can be extracted with standard ZIP extraction utilities such as WinZIP or WinRAR. When a BMZ file is extracted, the following files can be found:
BSP - The map file
TGA - Logo file
BNS - Portal Bonus Map Script File
How to Create and Install Bonus Map Files
The developer community at Valve Software has established guidelines for people who want to create and install Bonus Map files. It includes the following steps: