[BUGFIX] use absolute path for thumbnails

Classes/Controller/YoutubeController.php

@@ -137,7 +137,7 @@ class YoutubeController extends ActionController {
 			];
 			$imageService = GeneralUtility::makeInstance(ImageService::class);
 			$processedImage = $imageService->applyProcessingInstructions($fileObject, $processingInstructions);
-			$jsonArray[$index]['thumbnail'] = $imageService->getImageUri($processedImage);
+			$jsonArray[$index]['thumbnail'] = $imageService->getImageUri($processedImage, TRUE);
 		}
 
 		return $jsonArray;