style(*): Change index param type (not nullable) in a private method

Author: julienloizelet

src/Helper.php

@@ -49,7 +49,7 @@ private function buildRawBodyFromSuperglobals(
 
     private function appendFileData(
         array $fileArray,
-        ?int $index,
+        int $index,
         string $fileKey,
         string $boundary,
         int $threshold,