Extract WhatsApp thumbnails as Chat subitems if original media was not found in the case

[lfcnassif]

This could be very useful for CSAM and other cases.

Other approach would be to always set the thumb in the instant message item (this is commented out in code) so image/video messages could be viewed in the gallery, this is an old idea, the only drawback is increasing index size a bit. Some Image tasks would need to be updated to check for hasThumb() instead of isSupportedImage()