What's the exact error message?

If you download via BitTorrent, the files shouldn't be corrupted (unless you are trying to view the file without it downloading it completely). BitTorrent verifies files as they are downloaded in chunks. If you view the file before it finishes, it will, of course, tell you that the file is corrupted.

As for the file extension, it's usually *.avi (Windows A/V File)