It's the file format Warmerise uses for exported maps.
You'll need to download the WarmeriseMapExport package first. Once you have it installed, Unity will show up a new menu entry called "Warmerise". If you click on it and go to Custom Map > Export... (while having a Scene file selected), a [YourSceneName].wrm will get created in a folder named "__Maps".
Be sure to check out the tutorial.