A basic example of adding a file upload field to a Drupal form

gistfile1.php

<?php
/**
 * Implements hook_form_alter().
 */
function filetest_form_alter(&$form, &$form_state, $form_id) {
  switch ($form_id) {
    case 'contact_mail_page':
      // Need to set the enctype so the form will allow file uploads
      $form['#attributes']['enctype'] = "multipart/form-data";
      $form['new_file'] = array(
        '#type' => 'file',
        '#title' => t('New File'),
        '#description' => t('Select the file you want to attach.'),
      );
      $form['#submit'][] = 'filetest_submit';
      break;
  }
}

function filetest_submit($form, &$form_state) {
  $file = file_save_upload('new_file', array(), file_directory_path());
  // Print data so we can see what we've got
  //var_dump($file);
  //die();
  
  // Some of the values available from the $file object:
  $file_name = $file->filename; // "Letter.pdf"
  $file_path = $file->filepath; // "sites/default/files/Letter.pdf"
  $file_mime = $file->filemime; // "application/pdf"
  $file_size = $file->filesize; // "28711"
  $file_timestamp = $file->timestamp; // 1295668507
  $fid = $file->fid; // "15"
}