This imho is what most people[1][2] would expect to be the default in
modern times; considering this also cannot be "easily" configured
outside of the cli utilities gsettings or dconf it should just be the
default.
If there is any particular reason this has been false by default for such a long time, please let me know!
I can make a MR against gtk-3-24 branch as well later if this is accepted :)
[1] https://superuser.com/questions/843615/gtk-3-14-file-chooser-folders-first
[2] https://askubuntu.com/questions/789505/can-i-choose-to-not-sort-directories-before-files-on-file-open-dialogs