Revert to use null check so secondary method can be attempted

Thanks to @chrootchad

Author: stevebauman

src/Attachment.php

@@ -81,11 +81,7 @@ public function save(string $path): false|int
      */
     public function extension(): ?string
     {
-        if (! $this->filename) {
-            return null;
-        }
-
-        if ($ext = pathinfo($this->filename, PATHINFO_EXTENSION)) {
+        if ($ext = pathinfo($this->filename ?? '', PATHINFO_EXTENSION)) {
             return $ext;
         }