What format is "undf"?

"It's an undefined file, either a file which is not complete or not designed for your operating system.

Usually downloaders from BitTorrent sites will download a movie and try to preview it and receive this error. Just wait till the download completes."

"Well, they are undefined files. It's not a recognizable file format, they are some kind of corrupted files or incomplete files.

It's just a waste to keep on looking for the right codec to open such files. There aren't any. They are not audio nor video files. The solution? Delete the files and download them from a different site that you can play on your normal media players."

the gstreamer plugins, base, good, and bad worked for me. The fix was probably in only one of them, but I don't know which.