This was easy to do....
First download backup of your database file and copy/paste for safe editing.
Search for: ('osclass','allowedExt','png,gif,jpg,jpeg','STRING'),
and replace with: ('osclass','allowedExt','png,gif,jpg,jpeg,webp','STRING'),
Then clear database and upload edited file (make sure you uncheck Enable Foreign Key Field Checking)
OR more easy is open PhpMyAdmin - go to t_preference and edit table
osclass /allowedExt/ png,gif,jpg,jpeg with png,gif,jpg,jpeg,webp
In oc-includes/osclass look for file mimes.php and search for line: 'gif' => 'image/gif',
underneath that line add: 'webp' => 'image/webp',
Save file/Upload to server and your done.