Use dispatchSync instead of dispatchAfterResponse, the latter was too…

… slow sometimes

src/Mixins/UploadedFileMixin.php

@@ -57,7 +57,7 @@ public function save()
             Formats::dispatchGeneration($attachment);
 
             // Create the thumbnail now, so we don't have an empty preview in the next response
-            GenerateAttachmentFormat::dispatchAfterResponse(
+            GenerateAttachmentFormat::dispatchSync(
                 $attachment,
                 Thumbnail::make(),
             );