To use this file on your Nintendo 3DS, you must first extract the archive on your computer and then install the resulting .cia file using a modded console. 1. Extract the CIA File
The file is a compressed archive containing a "legit" CIA file for Luigi's Mansion: Dark Moon (LMDM) for the European (EUR) region.
Download and install an archive extractor like 7-Zip or WinRAR.