I attach a diff that describes a proposed fix.
The file dwm-portals.conf should go into debian/xdg-desktop-portal so as to be installed in /usr/share/xdg-desktop-portal
diff -r debian-8/desktop/dwm.desktop debian-9/desktop/dwm.desktop 6a7,14 > X-GNOME-WMName=dwm > DesktopNames=dwm > X-LightDM-DesktopName=dwm > > [Window Manager] > Name=dwm > SessionManaged=true > StartupNotification=false diff -r debian-8/dwm.install debian-9/dwm.install 1a2 > debian/xdg-desktop-portal/* /usr/share/xdg-desktop-portal Only in debian-9/: xdg-desktop-portal
[preferred] default=*
