[Silverstripe] Display ID and Filesize in AssetsAdmin (Works with and without DataObjectManager!)

_config.php

<?php 

if (class_exists('AssetManager')) DataObjectManager::allow_assets_override(false); // if DOM is installed
Object::add_extension('Folder', 'myAssetsFolderDecorator'); 
if (class_exists('AssetManager')) DataObjectManager::allow_assets_override(true); // if DOM is installed

myAssetsFolderDecorator.php

<?php

class myAssetsFolderDecorator extends DataObjectDecorator {
	public function updateCMSFields(Fieldset $fields) {
		$AssetsManager = $fields->fieldByName('Root')->fieldByName('Files')->fieldByName('Files');
		$AssetsManager->setFieldList(array('ID' => 'ID', 'Title' => 'Title', 'Filename' => 'Filename', 'getSize' => 'Size'));
		if (class_exists('AssetManager') && is_a($AssetsManager, 'AssetManager')) { 
			// if DOM is installed
			$AssetsManager->setColumnWidths(array(
				'ID' => 5,
				'Title' => 20,
				'Filename' => 65,
				'getSize' => 10
			));
		}
		return $fields;
	}
}