Drupal

  $form = array();
  // If this #attribute is not present, upload will fail on submit
  $form['#attributes']['enctype'] = 'multipart/form-data';
  $form['logotool_captions_upload'] = array(
    '#title' => t('Upload captions'),
    '#type'  => 'file',
    '#description' => t("...");

  $form['submit_upload'] = array('#type'  =>  'submit', '#value'  =>  'Upload caption list');

  return $form;
}

function logotool_captions_submit($form, &$form_state) {
  global $base_path;
  switch ($form_state['clicked_button']['#post']['op']) {
    case 'Upload caption list':
      $dest = file_directory_path() . '/' . LOGOTOOL_CWD; // not accepted!
      $validators = array();
      $file = file_save_upload('logotool_captions_upload', $validators, $dest);
      /* Let's just copy what that user.module does, which is to copy
       * the file out of the tmp directory to the real destination. From
       * that module:
       *     The image was saved using file_save_upload() and was
       *     added to the  files table as a temporary file. We'll make a
       *     copy and let the garbage collector delete the original upload.
       */
      // dest_filepath is path to a file, not to a directory.
     // Unlike file_save_upload.
      if ($file != 0) {
        $dest_filepath = file_directory_path() . '/' . variable_get('logotool_folder','logos') . '/' . LOGOTOOL_CAPTIONS_FILENAME;
        $result = file_copy($file->filepath, $dest_filepath, FILE_EXISTS_REPLACE);
        if ($result == 1) {
          drupal_set_message(t("%file uploaded successfully", array('%file' => $file->filename)));
        }
        else {
          // chmod 775; chown for apache (e.g., www, not user).
          form_set_error('logotool_captions', t("Failed to upload the logo; the %directory directory doesn't exist or is not writable.", array('%directory' => $dest_filepath)));
        }
      }
      else {
        form_set_error('logotool_captions', t("Failed to save the file."));
      }
      break;
  }
}