In WASM builds, passing "Images (*.png *.xpm *.jpg)" as the nameFilter argument to QFileDialog::getOpenFileContent (which is the example value from the documentation) has no effect and the resulting dialog does not have a filter. The issue here is almost certainly this code:

auto qtFilterStringToWebAcceptString = [](const QString &qtString) {
    // The Qt and Web name filter string formats are similar, but
    // not identical.
    return qtString.toStdString(); // ### TODO
};

I think that the "Web name filter string format" that the comment mentions is the format described at https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/file#accept, so that lambda expression presumably should convert "Images (*.png *.xpm *.jpg)" into something like ".png,.xpm,.jpg".