Looking into it more, I don’t think the preview widget is something a user can activate themselves. But if anyone with programming experience would like to investigate further, the following additional resources might help:
https://stackoverflow.com/questions/39522192/gtkfilechooserdialog-with-image-preview
https://docs.gtk.org/gtk3/iface.FileChooser.html
https://lazka.github.io/pgi-docs/Gtk-3.0/interfaces/FileChooser.html
https://valadoc.org/gtk±3.0/Gtk.FileChooser.html
https://www.gnu.org/software/guile-gnome/docs/gtk/html/GtkFileChooser.html
https://api.gtkd.org/gtk.FileChooserWidget.FileChooserWidget.html
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However, I did discover some other useful FileChooser tweaks and keyboard shortcuts along the way…
Open a terminal and run the following commands for each desired setting:
Sort directories first
gsettings set org.gtk.Settings.FileChooser sort-directories-first true
Set the window size
gsettings set org.gtk.Settings.FileChooser window-size '(900,450)'
Set the column to sort by
gsettings set org.gtk.Settings.FileChooser sort-column name|size|type|modified
Set the sort order
gsettings set org.gtk.Settings.FileChooser sort-order ascending|descending
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All of the above as well as other FileChooser settings can alternatively be set using dconf-editor as described here.
I.e. install and open dconf-editor, and then navigate to:
org → gtk → settings → file-chooser
For some apps that use the Gtk2 FileChooser (which doesn’t respect display scaling, e.g. KolourPaint), you need to instead change the settings here:
~/.config/gtk-2.0/gtkfilechooser.ini
And if any of your settings are getting reverted, you could try locking them with the following guide:
https://forum.xfce.org/viewtopic.php?pid=55276#p55276
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Keyboard shortcuts (worked out from here)
Enter location:
Ctrl + L
Jump to the Home folder:
Alt + Home
Jump to Desktop:
Alt + D
Jump to recently used files:
Alt + R
Jump to bookmark number 1, 2, 3,…,0:
Alt + 1, 2,…etc.
Up/Down a folder level:
Alt + Up/Down arrow key
Show hidden files (same as in Thunar etc.):
Ctrl + H
Search:
Alt + S