I think it's just too big for the forum to store its size, you could try to adjust the FILE_SIZE column to read an 8 vs a 6 but it would be an unsupported modification to the database or you could upload it to a section on your webspace and link to the file in your post.
Again, the database isn't designed to display files that large, you can either upload them to your webspace via FTP and link to it or adjust the int value in your database to be an 8 vs a 6.