Mark uploaded_file element as required

Explictly set the required attribute of the uploaded_file element of BpUploadForm to true.
previously the resulting-html element was not required and submitting the form caused null being passed.
This is necessary because this class does not extend the Web\Form,
and the Zend_Form does not mark the elment as required.

application/forms/BpUploadForm.php

@@ -142,6 +142,7 @@ class BpUploadForm extends BpConfigBaseForm
         /** @var \Zend_Form_Element_File $el */
         $el = $this->getElement('uploaded_file');
         $el->setValueDisabled(true);
+        $el->setAttrib('required', true);
 
         $this->setSubmitLabel(
             $this->translate('Next')