diff --git a/src/Mailer.php b/src/Mailer.php
index e8a3654..e11cbc9 100644
--- a/src/Mailer.php
+++ b/src/Mailer.php
@@ -70,6 +70,10 @@ public function send(Submission $submission, bool $runValidation = true): bool
         if ($submission->attachment !== null) {
             $allowedFileTypes = Craft::$app->getConfig()->getGeneral()->allowedFileExtensions;
 
+            if (!is_array($submission->attachment)) {
+                $submission->attachment = [$submission->attachment];
+            }
+
             foreach ($submission->attachment as $attachment) {
                 if (!$attachment) {
                     continue;
diff --git a/src/controllers/SendController.php b/src/controllers/SendController.php
index d263d5f..bc446b4 100644
--- a/src/controllers/SendController.php
+++ b/src/controllers/SendController.php
@@ -47,7 +47,7 @@ public function actionIndex()
             if (is_array($_FILES['attachment']['name'])) {
                 $submission->attachment = UploadedFile::getInstancesByName('attachment');
             } else {
-                $submission->attachment = [UploadedFile::getInstanceByName('attachment')];
+                $submission->attachment = UploadedFile::getInstanceByName('attachment');
             }
         }
 
diff --git a/src/models/Submission.php b/src/models/Submission.php
index 750b85d..2384974 100644
--- a/src/models/Submission.php
+++ b/src/models/Submission.php
@@ -39,8 +39,8 @@ class Submission extends Model
     public $message;
 
     /**
-     * @var UploadedFile[]|null[]|null
-     * @phpstan-var array<UploadedFile|null>|null
+     * @var UploadedFile|UploadedFile[]|null[]|null
+     * @phpstan-var UploadedFile|array<UploadedFile|null>|null
      */
     public $attachment;