They store micro-cached versions (thumbnails) of every photo and video on your phone.
tools extract image thumbnails from hidden Android cache files. thumbdata viewer free
files are index databases rather than standard images, you cannot open them by simply clicking them. Use these free tools instead: JustAnswer Thumbdata3 Viewer (Web-based) : A fully client-side HTML5 tool. You simply upload your .thumbdata They store micro-cached versions (thumbnails) of every photo