Hide files from WhatsApp's possible attachments
Settings &rarr; Apps &rarr; Then make sure you set the filter to see all applications (include system). 

Find Media Storage in the list and then press "clear data". Reboot. Wait for a while for database to regenerate.  That should fix the thumbnail problem. 

The new directory created should have .nomedia added to it or the folder name should start with a . (dot), before doing above step ,  else you will have the earlier problem of hiding photos