[REG 5.14] No files shown in filepicker

After https://codereview.qt-project.org/c/qt/qtwebengine/+/282057 it looks like the GTK filepicker doesn't show any files at all when trying to upload a file without an "accept" attribute.

 
This can be seen when starting simplebrowser with QT_QPA_PLATFORMTHEME=gtk3 and visiting https://mdn.github.io/learning-area/html/forms/file-examples/simple-file.html - the bottom right says "Accepted file types", but only directories (and not files) are displayed in the file picker.