I don't know of any movie-files being labelled BS.
Probably that is just a coincidence.
Are you sure that "pt1" is the file's extension?
Or is it simply meaning "part 1" (
probably packed/compressed) of a certain movie consisting of more parts.
Perhaps your Window's setting prevents the showing of so-called "known extensions" so that the actual file extension is hidden to you?
Maybe read item (3) of my Help2help-post.