/mod/assignment/type/uploadsingle/upload_form.php

<?php

// This file is part of Moodle - http://moodle.org/
//
// Moodle is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Moodle is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.

require_once($CFG->libdir.'/formslib.php');//putting this is as a safety as i got a class not found error.
/**
 * @package   mod-assignment
 * @copyright 1999 onwards Martin Dougiamas  {@link http://moodle.com}
 * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */
class mod_assignment_uploadsingle_form extends moodleform {
    function definition() {
        $mform = $this->_form;
        $instance = $this->_customdata;

        // visible elements
        //$mform->addElement('filemanager', 'newfile', get_string('uploadafile'));
        //$mform->addElement('filemanager', 'files_filemanager', get_string('uploadafile'), null, $instance['options']);
        $mform->addElement('filepicker', 'assignment_file', get_string('uploadafile'), null, $instance['options']);
        $mform->addRule('assignment_file', get_string('uploadnofilefound'), 'required', null, 'client');

        // hidden params
        $mform->addElement('hidden', 'contextid', $instance['contextid']);
        $mform->setType('contextid', PARAM_INT);
        $mform->addElement('hidden', 'userid', $instance['userid']);
        $mform->setType('userid', PARAM_INT);
        $mform->addElement('hidden', 'action', 'uploadfile');
        $mform->setType('action', PARAM_ALPHA);

        // buttons
        $this->add_action_buttons(true, get_string('savechanges', 'admin'));
    }
}