This is activated per default on mobile devices.
Contains a code copy from the xdgdesktopportal Qt platform theme.
The file can be found in
src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportalfiledialog.cpp
of the qtbase source tree.

A few fixes to the file were necessary to fix compilation with our compiler settings:
 * explicit QUrl from QString conversion
 * explicitly convert c string to QString
 * Don't initialize QString from nullptr, use {} instead

The portal forwarding can be activated by setting
the PLASMA_INTEGRATION_USE_PORTAL environment variable to 1