QXdgDesktopPortalFileDialog does not support preselecting and getting selected filter

When using the desktop portal, e.g. with environment variable QT_QPA_PLATFORMTHEME="xdgdesktopportal" set, preselecting a specific filter or retrieving the one that was selected does not work, i.e. the following methods don't work as expected:

QFileDialog::selectMimeTypeFilter
QFileDialog::selectedMimeTypeFilter
QFileDialog::selectNameFilter
QFileDialog::selectedNameFilter

A sample program (main.cxx) to test this is attached.

Steps to reproduce:

1. compile attach sample program
2. run it, using the portal file chooser:

QT_QPA_PLATFORMTHEME="xdgdesktopportal" ./main

3. check what filter is preselected in the first file dialog that opens
4. select PDF for the filter, then select a file and click OK
5. check output of the program
6. check what MIME filter is preselected in the second file dialog
7. select PDF for the filter, then select a file and click OK
8. check output of the program

Actual result:

in step 3): PDF filter is preselected
in step 5): output is: selectedNameFilter(): ""
in step 6: PDF filter is preselected
in step 8: output is: selectedMimeTypeFilter(): ""

Expected result:

The behavior should be the same as when using the "normal" (i.e. non-desktop) file chooser, i.e. without env variable QT_QPA_PLATFORMTHEME="xdgdesktopportal" set:

in step 3): text filter is preselected (since set with QFileDialog::selectNameFilter)
in step 5): output is: selectedNameFilter(): "PDF (*.pdf)"
in step 6: text filter is preselected (since set with QFileDialog::selectMimeTypeFilter("text/plain"))
in step 8: output is: selectedMimeTypeFilter(): "application/pdf"

Further notes:

• The corresponding functionality is implemented by the current_filter option in the FileChooser portal (input and output option/param of that name exists).
• The corresponding bug reports (with implementation in all involved components) are these:
  • https://gitlab.gnome.org/GNOME/gtk/-/issues/1492
  • https://gitlab.gnome.org/GNOME/gtk/-/issues/1820
• I plan to submit a patch to implement this.