protected function doCreateFileList($strFolder=null, $level=-1, $strFilter='')
	{
		// Deprecated since Contao 4.0, to be removed in Contao 5.0
		if ($strFilter === true)
		{
			@trigger_error('Passing "true" to Backend::doCreateFileList() has been deprecated and will no longer work in Contao 5.0.', E_USER_DEPRECATED);

			$strFilter = 'gif,jpg,jpeg,png';
		}

		$rootDir = System::getContainer()->getParameter('kernel.project_dir');
		$arrPages = scan($rootDir . '/' . $strFolder);

		// Empty folder
		if (empty($arrPages))
		{
			return '';
		}

		// Protected folder
		if (\in_array('.htaccess', $arrPages))
		{
			return '';
		}

		++$level;
		$strFolders = '';
		$strFiles = '';

		// Recursively list all files and folders
		foreach ($arrPages as $strFile)
		{
			if (strncmp($strFile, '.', 1) === 0)
			{
				continue;
			}

			// Folders
			if (is_dir($rootDir . '/' . $strFolder . '/' . $strFile))
			{
				$strFolders .=  $this->doCreateFileList($strFolder . '/' . $strFile, $level, $strFilter);
			}

			// Files
			else
			{
				// Filter images
				if ($strFilter != '' && !preg_match('/\.(' . str_replace(',', '|', $strFilter) . ')$/i', $strFile))
				{
					continue;
				}

				$strFiles .= sprintf('<option value="%s"%s>%s</option>', $strFolder . '/' . $strFile, (($strFolder . '/' . $strFile == Input::get('value')) ? ' selected="selected"' : ''), StringUtil::specialchars($strFile));
			}
		}

		if (\strlen($strFiles))
		{
			return '<optgroup label="' . StringUtil::specialchars($strFolder) . '">' . $strFiles . $strFolders . '</optgroup>';
		}

		return $strFiles . $strFolders;
	}