Filename extension of ARface database is undefined

When downloading the images of the AR face database from its original site: https://www2.ece.ohio-state.edu/~aleix/ARdatabase.html The images are stored in .raw format. Depending on which format was used to convert the files, different file name extensions need to be provided for the arface database interface: https://gitlab.idiap.ch/bob/bob.bio.face/-/blob/fdf454e829c320df1747cbbbcd050be9f6a26b34/bob/bio/face/config/database/arface.py#L10

Currently, this extension is fixed to .png. It would be optimal if we could have a resource parameter to update this extension in case people stored images in a different format -- as it is the case at Idiap, where .ppm was used.