Change file name to lower case

and codestyle
[MAILPOET-4203]

mailpoet/lib/Subscription/Captcha.php

@@ -113,7 +113,7 @@ class Captcha {
 
     $audio = null;
     foreach (str_split($captcha) as $character) {
-      $file = $audioPath . strtoupper($character) . '.mp3';
+      $file = $audioPath . strtolower($character) . '.mp3';
       if (!file_exists($file)) {
         throw new \RuntimeException("File not found.");
       }
@@ -127,6 +127,7 @@ class Captcha {
     header("Cache-Control: no-store, no-cache, must-revalidate");
     header('Content-Type: audio/mpeg');
 
+    //phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped, WordPressDotOrg.sniffs.OutputEscaping.UnescapedOutputParameter
     echo $audio;
     exit;
   }