A bit of history:
Before Kicad v4 the footprint library has been implemented similary to how the symbol libs are still implemented. There was one big file per library containing all footprints of this library.
This file was called .mod
Along came the new file format where the library is a directory containing one file per footprint.
The directory is called library_name.pretty, the footprints are called footprint_name.kicad_mod
Something similar will be done for the symbol libraries. I think the library directory will have the .sweet name. I am not sure how the symbol files will be called. Maybe .kicad_sym (I'm sure this information can be found somwhere)
As i explained above the .pretty "file" is a directory. You can open it in any file browser to see the .kicad_mod files within it.
To be able to use this library you need to add it to the fp-lib-table.
Open pcb_new and go to preferences->footprint library wizard.
There select files on my computer and navigate to the .pretty directory.
Follow the instructions of the wizard. (If you have problems there, just ask.)
There are filter settings that determine what is displayed in the rightmost column.