Fix decode filename (Webklex#535)

Author: grnsv

src/Attachment.php

@@ -307,6 +307,7 @@ public function decodeName(?string $name): string {
             if (str_contains($name, "''")) {
                 $parts = explode("''", $name);
                 if (EncodingAliases::has($parts[0])) {
+                    $encoding = $parts[0];
                     $name = implode("''", array_slice($parts, 1));
                 }
             }
@@ -323,6 +324,10 @@ public function decodeName(?string $name): string {
                 $name = urldecode($name);
             }
 
+            if (isset($encoding)) {
+                $name = EncodingAliases::convert($name, $encoding);
+            }
+
             // sanitize $name
             $replaces = [
                 '/\\\\/' => '',