QImageReader::imageFormat(const QString &) cannot autodetect image format

I have a file 1.jpg.
I rename it to 1.bmp.
QImageReader::imageFormat(file) will return an empty QByteArray.

That's because:
In function createReadHandlerHelper() in file "qimagereader.cpp"
From line 308 to line 319
It will use keys.at(i) to create QImageIOPlugin.
However, while testing whether the plugin can read, it use testFormat rather than keys.at(i)

If I change testFormat from "bmp" to "jpg" while debuging, the desired result will be returned.

Besides, keys won't contain "png" and "bmp" which make it cannot autodetect those formats.