core-bundle/src/Resources/contao/dca/tl_module.php
public function setMultiSrcFlags($varValue, Contao\DataContainer $dc)
	{
		if ($dc->activeRecord && $dc->activeRecord->type == 'randomImage')
		{
			$GLOBALS['TL_DCA'][$dc->table]['fields'][$dc->field]['eval']['isGallery'] = true;
			$GLOBALS['TL_DCA'][$dc->table]['fields'][$dc->field]['eval']['extensions'] = Contao\Config::get('validImageTypes');
		}

		return $varValue;
	}
public function setMultiSrcFlags($varValue, Contao\DataContainer $dc)
	{
		if ($dc->activeRecord && $dc->activeRecord->type == 'randomImage')
		{
			$GLOBALS['TL_DCA'][$dc->table]['fields'][$dc->field]['eval']['isGallery'] = true;
			$GLOBALS['TL_DCA'][$dc->table]['fields'][$dc->field]['eval']['extensions'] = Contao\Config::get('validImageTypes');
		}

		return $varValue;
	}
Dynamically add flags to the "multiSRC" field

@param mixed                $varValue
@param Contao\DataContainer $dc

@return mixed