If you add an Youtube Video with the File-Module, the Generation of the Preview Image is broken and no Preview-Image is created.
There is a Part missing on top in the method FileProcessingService for TYPO3 8.7 and 9.5.
public function processFile(
Resource\FileInterface $fileObject,
Resource\ResourceStorage $targetStorage,
$taskType,
$configuration
) {
// Enforce default configuration for preview processing here,
// to be sure we find already processed files below,
// which we wouldn't if we would change the configuration later, as configuration is part of the lookup.
if ($taskType === Resource\ProcessedFile::CONTEXT_IMAGEPREVIEW) {
$configuration = Resource\Processing\LocalPreviewHelper::preProcessConfiguration($configuration);
}