This is useful to be able to support file formats other than image
formats. In particular I will use this in the next commit to support a
"GIMP extension" format. When GIMP will open such file, it will
install an extension (not open an image on canvas).

This is an internal flag only, i.e. only usable from core GIMP. File
formats which a plug-in can register are still only image file formats.