1) Open http://doc.qt.io/qt-4.8/qimage.html, read "If the format is not specified (which is the default), the loader probes the file for a HEADER to guess the file format.".
2) Load any jpg file, and rename to "test.png".
3) Run simple test:

#include<QImage>
#include<cassert>

int main(int argc, char *argv[])
{
    QImage test("test.png");
    assert(!test.isNull());
    return 0;
}

In result - SIGABRT. If rename file to "test.jpg" or "test", SIGABRT signal don't show. In other words, if file have extension, QImage ignore file header. And manual don't have a single word about it.