Extend a Varien Form Element for a Custom Module - magento

Question

Improving on this question:
Is it good practice to add own file in lib/Varien/Data/Form/Element folder
The accepted answer shows how to extend a Varien form element, but this will not work if you want to package it into a custom module.
What would be the proper method of extending the Varien form element in a module? A simple XML setting I'm hoping?
Update:
Thanks Vinai for the response. Although that does work, I was hoping to extend the form element somehow. My extension is using the base File form element to allow administrators to upload files to categories. So, I'm not directly adding the form elements to the fieldset myself.
I suppose it's possible to to check for the file input on my category block on the backend: Mage_Adminhtml_Block_Catalog_Category_Tab_Attributes , and then change the form element if it is 'file' to 'mycompany_file' -- but this seems like a workaround.
Is there an easier way? Thanks again Vinai.

Answer 1

On the Varien_Data_Form instance you can specify custom element types like this:
$fieldset->addType('custom', 'Your_Module_Model_Form_Element_Custom');
Then, add your element with
$fieldset->addField('the_name', 'custom', $optionsArray);
If you are using a form without fieldsets you can do the same on the Varien_Data_Forminstance, too.
EDIT: Expand answer because of new additional details in the question.
In the class Mage_Adminhtml_Block_Widget_Form::_setFieldset() there is the following code:
$rendererClass = $attribute->getFrontend()->getInputRendererClass();
if (!empty($rendererClass)) {
$fieldType = $inputType . '_' . $attribute->getAttributeCode();
$fieldset->addType($fieldType, $rendererClass);
}
Because of this the attribute frontend_input_renderer on the attributes can be used to specify custom element classes.
This property can be found in the table catalog_eav_attribute, and luckily enough it isn't set for any of the category image attributes.
Given this, there are several ways to apply customizaton.
One option is to simply set the element class in the table using an upgrade script.
Another would be using an observer for the eav_entity_attribute_load_after event and setting the input renderer on the fly if the entity_type_id and the input type matches.
So it is a little more involved then just regular class rewrites in Magento, but it is quite possible.

Answer 2

You don't necessarily need to have a file in the lib/Varien/ directory in order to extend it. If you needed to add an element to that collection, you should be able to extend one of the Elements in your app/code/local module. The answer to the question you referenced seems to also indicate this is the case. I would create your custom field, extending its highest-level function set (i.e., lib/Varien/Data/Form/Element/File.php).
If you want to override the Mage_Adminhtml_Block_Catalog_Category_Tab_Attributes block, then you should extend that block in your module and then reference the new one. You may wish to extend the block using an event observer rather than an XML rewrite, for compatibility purposes.