Had the same problem. A pain.
I renamed the clip in the organizer window to something comprehendible (Birthday_cake.MTS!)
You could export it to some other format (MPEG2) that shows the thumbnail and then import it if you want a longer approach.
PE8 does not show thumbnails of MTS files in the Elements Organizer. However you can see the thumbnail in PRE organizer. However you can work with mts file as wished ie play ,export etc without losing any quality