unable to start mongod - Fatal assertion 28579 UnsupportedFormat
mongod -vvvvvv -dbpath /var/lib/mongodb


and remove corrupted datafile.

rm f index-1234-123456789123456789.wt


and then repair using following command 

mongod --repair --dbpath /var/lib/mongodb/ --storageEngine wiredTiger


Note: Have a backup of /var/lib/mongodb