Qt file dialogs under GNOME and MATE

Now for the file extensions: the filter I pass to the "save file" dialog is "Molsketch default (*.msk *.msm)". On my system (KDE, OpenSUSE 15.0 or 42.3, depending on the machine), this is shown as "Molsketch default" and there is a checkbox for appending the default extension (.msk). For the "save a molecule" dialog, the same applies for ".msm". Now, when I try the same thing on Fedora (Gnome, I guess), it also shows "Molsketch default" (so, without the extension) for the filter in the dialog. When I save something, however, it does not add any extension. Comparing that to the default text editor (gedit), this seems to be expected behavior: I can select "All Text Files" and there will be no extension appended (on the other hand, that really seems to be what the filter selects -- not *.txt, as I would have expected). In LibreOffice, it seems that the extension is shown in the filter text and appended to the name if no extension is given. On my system, I can simply pass "Molsketch default (*.msk) (*.msk *.msm)" to have "Molsketch default (*.msk)" displayed. On Fedora, this seems to get converted to "Molsketch default", however. Oh, and the "add '*.msk' automatically" functionality worked due to a bizarre piece of code that disassembled the filter string and thus extracted the extension. That was added if no extension was given in filename returned by the dialog. Should that happen instead (of course without the awkward parsing of the filter)? I'm a little confused here by the difficulties to use the native save dialog via Qt. Any suggestions?