Backport da0fa0f6: Unbreak svg and video thumbnails

BUG: 280879

lib/thumbnailloadjob.cpp

@@ -667,7 +667,10 @@ void ThumbnailLoadJob::checkThumbnail() {
 		KFileItemList list;
 		list.append(mCurrentItem);
 		const int pixelSize = ThumbnailGroup::pixelSize(mThumbnailGroup);
-		KIO::Job* job = KIO::filePreview(list, QSize(pixelSize, pixelSize));
+		if (mPreviewPlugins.isEmpty()) {
+			mPreviewPlugins = KIO::PreviewJob::availablePlugins();
+		}
+		KIO::Job* job = KIO::filePreview(list, QSize(pixelSize, pixelSize), &mPreviewPlugins);
 		//job->ui()->setWindow(KApplication::kApplication()->activeWindow());
 		connect(job, SIGNAL(gotPreview(const KFileItem&, const QPixmap&)),
 			this, SLOT(slotGotPreview(const KFileItem&, const QPixmap&)) );

lib/thumbnailloadjob.h

@@ -218,6 +218,8 @@ private:
 
 	ThumbnailThread mThumbnailThread;
 
+	QStringList mPreviewPlugins;
+
 	void determineNextIcon();
 	void startCreatingThumbnail(const QString& path);