minor fix

app/Http/Controllers/Api/Assets/UploadFileController.php

@@ -165,6 +165,7 @@ class UploadFileController extends Controller {
         if (!array_key_exists($mime, $this->validMimes)) {
             throw new StoreResourceFailedException('Validation Error', [
                 'Content-Type' => 'The Content Type sent is not valid',
+                'mime' => $mime
             ]);
         }
     }