Drupal: Admin form for file

gistfile1.php

<?php

function mymodule_admin_image() {
  $form = array();
  
  // Image wrapper
  $form['image_default'] = array(
    '#type'=>'fieldset',
    '#title' => "Image par defaut",
    '#collapsible' => TRUE,
    '#collapsed' => FALSE,
  );

    // Image preview
    $form['image_default']['mymodule_image_default_preview'] = array(
      '#markup' => theme(
        'image_style',
        array(
          'style_name' => 'medium',
          'path' => variable_get('mymodule_image_default', "")
        )
      ),
    );

    // Image path
    $form['image_default']['mymodule_image_default'] = array(
      '#type' => 'textfield',
      '#disabled' => TRUE,
      '#size' => 25,
      '#default_value' => variable_get('mymodule_image_default', ""),
    );

    // Image upload file
    $form['image_default']['mymodule_image_default_upload'] = array(
      '#type' => 'file',
      '#maxlength' => 40,
    );
  
  
  $form = system_settings_form($form);
  // We don't want to call system_settings_form_submit(), so change #submit.
  array_pop($form['#submit']);
  $form['#submit'][] = 'mymodule_admin_image_submit';
  $form['#validate'][] = 'mymodule_admin_image_validate';
  return $form;
}


function mymodule_admin_image_validate($form, &$form_state) {
  // Handle file uploads.
  $validators = array(
    'file_validate_is_image' => array(),
    'file_validate_extensions' => array('jpg jpeg'),
  );


  $file = file_save_upload('mymodule_image_default_upload', $validators);
  if (isset($file)) {
    if ($file) {
      $form_state['values']['mymodule_image_default_upload'] = $file;
    }
    else {
      form_set_error('mymodule_image_default_upload', "Error");
    }
  }
  
} // mymodule_admin_image_validate


/**
 * Process system_theme_settings form submissions.
 */
function mymodule_admin_image_submit($form, &$form_state) {
  $values = $form_state['values'];

  if ($file = $values['mymodule_image_default_upload']) {
    unset($values['mymodule_image_default_upload']);
    variable_set('mymodule_image_default', file_unmanaged_copy($file->uri));
  }

  drupal_set_message(t('The configuration options have been saved.'));
  //cache_clear_all();
} // mymodule_admin_image_submit