The maintainer of the Magic Mirror has a limit on uploads of 2-3MB per file. Harri broke up the design files into two zips. Each zip also has a TXT file associated with it. Download all 4 files and unzip both ZIPs into a folder that ends with '.dsn', as in 'LastDays.dsn'. If the folder is under the UAF install folder you will be able to open it and edit it. If not placed under UAF, you can still put a UAFWin.exe into the dsn folder and play the design.
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CocoaSpud
Dungeon Craft Development Team
http://uaf.sourceforge.net