FileInfo.com - The File Extensions Resource

.LXF File Extension

File Type	LEGO Digital Designer Model File
File Format	File
Common?	No
File Description	3D model format used by LEGO Digital Designer (LDD), a virtual LEGO building program; saved in a compressed .ZIP format containing three files: MODEL100.MODEL, IMAGE100.PNG, and IMAGE100.LXFML.
Program(s) that open lxf files	Mac / Windows: LEGO Digital Designer; BricksViewer

Information from FileInfo.com - The File Extensions Resource.
http://www.fileinfo.com/extension/lxf