You made a form in adminhtml with upload file but in the controller the $_FILES var is hopelessly empty ?
You probably forgot to add “‘enctype’ => ‘multipart/form-data’,” on your form declaration
example :
file : /app/code/local/Adin/Shipping/Block/Adminhtml/Delivery/Cost/Import/Edit/Form.php
protected function _prepareForm()
{
$form = new Varien_Data_Form(array(
'id' => 'edit_form',
'action' => $this->getUrl('*/*/save', array('id' => $this->getRequest()->getParam('id'))),
'method' => 'post',
'enctype' => 'multipart/form-data',
));
$fieldset->addField('fichier', 'file', array(
'label' => $helper->__('Data file'),
'value' => 'upload',
'name' => 'fichier',
'after_element_html' => $helper->__('<small>File from carrier</small>'),
'required' => true,
));