code: protected function getPreviewImage(File $objFile, $strInfo, $strClass='gimage')
  	{
  		if (($objFile->isSvgImage || ($objFile->height <= Config::get('gdMaxImgHeight') && $objFile->width <= Config::get('gdMaxImgWidth'))) && $objFile->viewWidth && $objFile->viewHeight)
  		{
  			// Inline the image if no preview image will be generated (see #636)
  			if ($objFile->height !== null && $objFile->height <= 75 && $objFile->width !== null && $objFile->width <= 100)
  			{
  				$image = $objFile->dataUri;
  			}
  			else
  			{
  				$rootDir = System::getContainer()->getParameter('kernel.project_dir');
  				$image = System::getContainer()->get('contao.image.image_factory')->create($rootDir . '/' . $objFile->path, array(100, 75, ResizeConfiguration::MODE_BOX))->getUrl($rootDir);
  			}
  		}
  		else
  		{
  			$image = Image::getPath('placeholder.svg');
  		}
  
  		if (strncmp($image, 'data:', 5) === 0)
  		{
  			return '<img src="' . $objFile->dataUri . '" width="' . $objFile->width . '" height="' . $objFile->height . '" alt="" class="' . $strClass . '" title="' . StringUtil::specialchars($strInfo) . '">';
  		}
  
  		return Image::getHtml($image, '', 'class="' . $strClass . '" title="' . StringUtil::specialchars($strInfo) . '"');
  	}
docs: Return the preview image
  
  @param File   $objFile
  @param string $strInfo
  @param string $strClass
  
  @return string