code: public function checkFilename($varValue, Contao\DataContainer $dc)
  	{
  		$varValue = str_replace('"', '', $varValue);
  
  		if (strpos($varValue, '/') !== false || preg_match('/\.$/', $varValue))
  		{
  			throw new Exception($GLOBALS['TL_LANG']['ERR']['invalidName']);
  		}
  
  		// Check the length without the file extension
  		if ($dc->activeRecord && $varValue != '')
  		{
  			$intMaxlength = $GLOBALS['TL_DCA'][$dc->table]['fields'][$dc->field]['eval']['maxlength'];
  
  			if ($dc->activeRecord->type == 'file')
  			{
  				$intMaxlength -= (\strlen($dc->activeRecord->extension) + 1);
  			}
  
  			if ($intMaxlength && Patchwork\Utf8::strlen($varValue) > $intMaxlength)
  			{
  				throw new Exception(sprintf($GLOBALS['TL_LANG']['ERR']['maxlength'], $GLOBALS['TL_DCA'][$dc->table]['fields'][$dc->field]['label'][0], $intMaxlength));
  			}
  		}
  
  		return $varValue;
  	}
docs: Check a file name and romanize it
  
  @param string                                $varValue
  @param Contao\DataContainer|Contao\DC_Folder $dc
  
  @return mixed
  
  @throws Exception