Fix: issue 132 - pathinfo(): Passing null to parameter #1 () of type string is deprecated

Author: chrootchad

src/Attachment.php

@@ -81,7 +81,7 @@ public function save(string $path): false|int
      */
     public function extension(): ?string
     {
-        if ($ext = pathinfo($this->filename, PATHINFO_EXTENSION)) {
+        if ($ext = pathinfo($this->filename ?? '', PATHINFO_EXTENSION)) {
             return $ext;
         }