Is it permissible to download a video from a forum when it is unknown whether or not it is copyrighted?

If there is nothing to prevent the download of the material, and it has not been tampered with, and its content is permissible, then there is no harm in downloading it and benefiting from it. The default is the permission of people for others to benefit from what is published on websites and forums, and customary permission suffices, as it takes the place of actual permission.