QDesktopServices backend on linux?

Question

What is used to open urls with QDesktopServices::openUrl(...)?

xdg-open, kopen, selmademachanism or a mix of them? I ask because it fails on a scheme where xdg-open does not.

Edit: okay its getting even more curious: The missing scheme works in debug mode, but not in release mode...

Example code:

#include <QApplication>
#include <QDesktopServices>
#include <QUrl>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QDesktopServices::openUrl(QUrl("tg://resolve?domain=imagebot"));
}