If the server is using a standard path prefix and a standard file layout and is using standard file names it isn't that difficult to find the location of a media file and then from there it would be easier to find bore files, assuming the paths are consistent.
But even for low entropy strings, long strings are difficult to brute force, and rainbow tables are useless for this use case.
That is possible, but I don't think you need to worry about that. Having a copy of a movie is not normally itself a crime.