Uhm ... probably you're right but look at qt's doc (4.5):

QPluginLoader::QPluginLoader ( const QString & fileName, QObject * parent = 0 )

Constructs a plugin loader with the given parent that will load the plugin specified by fileName.

To be loadable, the file's suffix must be a valid suffix for a loadable library in accordance with the platform, e.g. .so on Unix, - .dylib on Mac OS X, and .dll on Windows. The suffix can be verified with QLibrary::isLibrary().
I understand that the plugin is loaded by the constructor so, if it fails here, why does it work in the method instance()