edit

src/Store/Local.php

@@ -160,6 +160,8 @@ class Local extends Core implements Config
 			$this->ext = '.webp';
 			$this->name .= '_wp' . $num;
 			$dest = $this->file();
+		} else {
+			$dest .= '.webp';
 		}
 
 		$this->img()->setType('im');