diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0c8c4ba..9f173c0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,7 @@
 
 - Added Craft 5 compatibility.
 - Added a missing Dutch translation. ([#257](https://github.com/craftcms/contact-form/issues/257))
+- Fixed a bug where it wasn’t possible to upload a single file with the `attachment` param. ([#254](https://github.com/craftcms/contact-form/issues/254))
 
 ## 3.0.1 - 2023-03-16
 
@@ -22,7 +23,7 @@
 
 ## 2.5.2 - 2023-03-16
 
-- Added translations for for `Email` and `Name`. ([#235](https://github.com/craftcms/contact-form/issues/235))
+- Added translations for `Email` and `Name`. ([#235](https://github.com/craftcms/contact-form/issues/235))
 
 ## 2.5.1 - 2022-05-02
 
diff --git a/README.md b/README.md
index 5e2ba16..48c6aca 100644
--- a/README.md
+++ b/README.md
@@ -169,7 +169,7 @@ return [
 
 ### Overriding plugin settings
 
-If you create a [config file](https://craftcms.com/docs/4.x/config/general.html) in your `config/` folder called `contact-form.php`, you can override
+If you create a [config file](https://craftcms.com/docs/4.x/config/) in your `config/` folder called `contact-form.php`, you can override
 the plugin’s settings in the Control Panel.  Since that config file is fully [multi-environment](https://craftcms.com/docs/4.x/config/#multi-environment-configs) aware, this is
 a handy way to have different settings across multiple environments.
 
diff --git a/src/Mailer.php b/src/Mailer.php
index 611a778..48f2a84 100644
--- a/src/Mailer.php
+++ b/src/Mailer.php
@@ -71,6 +71,10 @@ 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 cff7484..fe31a19 100644
--- a/src/controllers/SendController.php
+++ b/src/controllers/SendController.php
@@ -46,7 +46,7 @@ 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;