There is only one file type that uses the .NETSPD extension:
NetSpot Survey Project File
Project file created by NetSpot, a program used for surveying and visualizing wireless networks; can store multiple network maps, each of which are overlaid with detected wireless information; can be used for planning as well as creating wireless heatmaps in homes and other buildings.
NOTE: You should use a portable computer when surveying wireless networks with NetSpot. This allows you to be mobile while you map your wireless network and capture locations where signal strengths and weaknesses are present.
Updated: January 11, 2012