Fantasy Grounds uses XML files and png/jpg/webp image files.
The XML files often contain Lua also.
They are generally packaged up for easy distribution into
.pak files for rulesets
.ext files for extensions
.mod files for modules
.ppk files for portraits
The packages however are just renamed ZIP files.
Take not that the file structure of each type is also important and when you ZIP up you should use standard ZIP and you package up the contents - not the parent sub folder.
If you install 7zip it will be able to open all these file types easily.

