thumbnail/imageplugin: add image/x-adobe-dng support
Since image/x-adobe-dng inherits from mime image/x-dcraw, prevents the rawthumbnail plugin in kdegraphics-thumbnailers to render it, since this one cannot render those files.
Since image/x-adobe-dng inherits from mime image/x-dcraw, prevents the rawthumbnail plugin in kdegraphics-thumbnailers to render it, since this one cannot render those files.