Magento allows us to create custom form in admin panel by creating our own custom module. We can create almost any field in the admin form by creating a class that extends the base class Mage_Adminhtml_Block_Widget_Form. The _prepareForm method allows to define our form elements.
Sometimes we may need to upload a file in the adminform, the default file field doesn’t provide the image preview when we upload an image file. For this purpose we need to use image field type as shown in the below snippet
$fieldset->addField('filename', 'image', array( 'label' => Mage::helper('customform')->__('Image'), 'required' => false, 'name' => 'filename', ));
