$request = $event->getRequest();
print_r($request->files);die;
gives me
Symfony\\Component\\HttpFoundation\\FileBag Object
(
When you upload files you get UploadFile
(API link) objects (basically the wrappers of array).
$this->getRequest()->files[0]->getClientOriginalName();
Can't try this now but you might need to do this instead:
$this->getRequest()->files['name_of_file_field_in_post']->getClientOriginalName();
where you would replace name_of_file_field_in_post
with your form field's name.