Add correct extension lookup to getExtension

Previous change corrected the getSupportedExtensions function, this
change fixes the getExtension function.

Change-Id: Ie4d36366d3261bd65c7d5f6677902781156f9ad2
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@theqtcompany.com>