rauldipeas that's a lot to send through. I've set the limit to 200 MB, which I feel like is maximum that we're going to support. The limit is set for all forums, so with some effort, someone could halt our disk space easily, if the limit was even higher. Remember that we're a free service not centered around file uploads.
If your forum is based around frequent file uploads, I'd suggest you to host it by yourself instead. There's not much that you'll get from a free service like FreeFlarum when it comes to uploading. Or, you can just upload the file outside of FreeFlarum and then share a link to that file in a post.