Magento – enctype => ‘multipart/form-data’ not working

I am attaching an image with tags in admin panel and trying to save tag id and image name in my custom table and also the image in media/tagimage folder.

I have put a button to upload the image in Mage_Adminhtml_Block_Widget_Form.

Here is my block code:

protected function _prepareForm()
{
    $model = Mage::registry('tag_tag');

    $form = new Varien_Data_Form(
        array('id' => 'edit_form',
            'action' => $this->getData('action'),
            'method' => 'post',
            'enctype' => 'multipart/form-data')
    );

    $fieldset = $form->addFieldset('base_fieldset',
        array('legend'=>Mage::helper('tag')->__('General Information')));

    if ($model->getTagId()) {
        $fieldset->addField('tag_id', 'hidden', array(
            'name' => 'tag_id',
        ));
    }

    $fieldset->addField('form_key', 'hidden', array(
        'name'  => 'form_key',
        'value' => Mage::getSingleton('core/session')->getFormKey(),
    ));

    $fieldset->addField('store_id', 'hidden', array(
        'name'  => 'store_id',
        'value' => (int)$this->getRequest()->getParam('store')
    ));

    $fieldset->addField('tag_name', 'text', array(
        'name' => 'tag_name',
        'label' => Mage::helper('tag')->__('Tag Name'),
        'title' => Mage::helper('tag')->__('Tag Name'),
        'required' => true,

    ));

    //custom
    $fieldset->addField('tag_image', 'file', array(
        'name' => 'tag_image',
        'label' => Mage::helper('tag')->__('Tag Image'),
        'title' => Mage::helper('tag')->__('Tag Image'),
    ));
    //end custom

    $fieldset->addField('status', 'select', array(
        'label' => Mage::helper('tag')->__('Status'),
        'title' => Mage::helper('tag')->__('Status'),
        'name' => 'tag_status',
        'required' => true,
        'options' => array(
            Mage_Tag_Model_Tag::STATUS_DISABLED => Mage::helper('tag')->__('Disabled'),
            Mage_Tag_Model_Tag::STATUS_PENDING  => Mage::helper('tag')->__('Pending'),
            Mage_Tag_Model_Tag::STATUS_APPROVED => Mage::helper('tag')->__('Approved'),
        ),
        'after_element_html' => ' ' . Mage::helper('adminhtml')->__('[GLOBAL]'),
    ));

    $fieldset->addField('base_popularity', 'text', array(
        'name' => 'base_popularity',
        'label' => Mage::helper('tag')->__('Base Popularity'),
        'title' => Mage::helper('tag')->__('Base Popularity'),
        'after_element_html' => ' ' . Mage::helper('tag')->__('[STORE VIEW]'),
    ));


    if (!$model->getId() && !Mage::getSingleton('adminhtml/session')->getTagData() ) {
        $model->setStatus(Mage_Tag_Model_Tag::STATUS_APPROVED);
    }

    if ( Mage::getSingleton('adminhtml/session')->getTagData() ) {
        $form->addValues(Mage::getSingleton('adminhtml/session')->getTagData());
        Mage::getSingleton('adminhtml/session')->setTagData(null);
    } else {
        $form->addValues($model->getData());
    }

    $this->setForm($form);
    return parent::_prepareForm();
}

and this is my code in saveAction in tagController for saving the image:

$customPostData = $this->getRequest()->getPost('tag_image');
  //print_r($customPostData); die();

   $name =  $_FILES["tag_image"]["name"];
        $path =  $_FILES["tag_image"]["tmp_name"];
        $media_path = Mage::getBaseDir('media')."/tagimage";
        move_uploaded_file($path,$media_path."/".$name);
  $tagimage_collection = Mage::getModel('sports365/tagimage');
  $tagimage_collection->setTagImage($name)
                      ->setTagId($customPostData['tag_id']);

Now my problem is that even I have $customPostData = $this->getRequest()->getPost('tag_image');
  //print_r($customPostData); die();

   $name =  $_FILES["tag_image"]["name"];
        $path =  $_FILES["tag_image"]["tmp_name"];
        $media_path = Mage::getBaseDir('media')."/tagimage";
        move_uploaded_file($path,$media_path."/".$name);
  $tagimage_collection = Mage::getModel('sports365/tagimage');
  $tagimage_collection->setTagImage($name)
                      ->setTagId($customPostData['tag_id']);

My problem is even I have added enctype => 'multipart/form-data' in form initialising but it still not working and nothing is coming in $_FILES["tag_image"]["name"];

Now if I go to inspect element,edit as html and add enctype => 'multipart/form-data' to the form, my code will save the image both in media folder and database.

Note: Right now I am testing in core files