That message means that Spark will wait for another client to become online and then will try to send a file again. It doesn’t mean the file will be stored somewhere on the server.
File transfer problems between versions or in 2.8.3 is separate and complex issue. In short, file transfer is broken in 2.8.3, there are no developers working on Spark, so it probably won’t be fixed soon. If file transfer is very important for you, look into other clients.