the extension's file extension is .ext. The file contents are XML. Sorry, I am a developer, so I sometimes forget that people do not know what XML is.
From reading, it appears that .ext files for Fantasy Grounds can contain a zip archive (renamed to ".ext") or an XML file (renamed to ".ext")
However none of this helps me, unfortunately, as the DOE extensions I downloaded and put in the correct places are not showing in my demo copy of Fantasy Grounds. I am assuming it is my problem, and not that of the extension, but I have no clue why.