Fixed web submission if keeping required file.

forms/submission_form.php

@@ -87,6 +87,8 @@ class mod_vpl_submission_form extends moodleform {
                 $mform->addElement( 'radio', $field . 'action', '',
                         get_string( 'replacefile', VPL ), 'replace' );
                 $mform->disabledIf( $field . 'action', 'submitmethod', 'neq', 'files' );
+                $mform->addElement( 'hidden', $field . 'name', $reqfile );
+                $mform->setType( $field . 'name', PARAM_RAW );
             }
             $mform->addElement( 'filepicker', $field );
             $mform->disabledIf( $field, 'submitmethod', 'neq', 'files' );