code: protected function checkValue($varInput)
  	{
  		if ($varInput == '')
  		{
  			return;
  		}
  
  		if (strpos($varInput, ',') === false)
  		{
  			$arrUuids = array($varInput);
  		}
  		else
  		{
  			$arrUuids = array_filter(explode(',', $varInput));
  		}
  
  		$objFiles = FilesModel::findMultipleByUuids($arrUuids);
  
  		if ($objFiles === null)
  		{
  			return;
  		}
  
  		$rootDir = System::getContainer()->getParameter('kernel.project_dir');
  
  		foreach ($objFiles as $objFile)
  		{
  			// Only files can be selected
  			if ($this->filesOnly && is_dir($rootDir . '/' . $objFile->path))
  			{
  				$this->addError($GLOBALS['TL_LANG']['ERR']['filesOnly']);
  				break;
  			}
  
  			// Only folders can be selected
  			if ($this->files === false && !is_dir($rootDir . '/' . $objFile->path))
  			{
  				$this->addError($GLOBALS['TL_LANG']['ERR']['foldersOnly']);
  				break;
  			}
  
  			// Only files within a custom path can be selected
  			if ($this->path && strpos($objFile->path, $this->path . '/') !== 0)
  			{
  				$this->addError(sprintf($GLOBALS['TL_LANG']['ERR']['pathOnly'], $this->path));
  				break;
  			}
  
  			// Only certain file types can be selected
  			if ($this->extensions && !is_dir($rootDir . '/' . $objFile->path))
  			{
  				$objFile = new File($objFile->path);
  				$extensions = StringUtil::trimsplit(',', $this->extensions);
  
  				if (!\in_array($objFile->extension, $extensions))
  				{
  					$this->addError(sprintf($GLOBALS['TL_LANG']['ERR']['extensionsOnly'], $this->extensions));
  					break;
  				}
  			}
  		}
  	}
docs: Check the selected value
  
  @param mixed $varInput