@garylue,
Please note the server size is the same in each place. Just in one place, it set via KB, in the second place is via MB, thus the values are different.
Thank you for your reply. I am curious that if the server limit is 64 mb, why am I allowed 800 mb in the base WPForo but not Advanced Attachments?
The wpForo Advanced Attachments designed to work in the following way. It doesn't allow to set the value high then the server limit. Even if you increase the values, the server doesn't allow you to upload the file highest then the server size.
However, the default wpForo didn't configure in this way. So you can set the value highest then the server value, but you can't upload the file highest then the server size, in this case too. Also as you can see in the screenshot provided below, it says "You can not set this value more than "upload_max_filesize" and "post_max_size". If you want to increase server parameters please contact your hosting service support."
https://www.screencast.com/t/CNYLLPrL
So please contact your hosting service support and ask them to increase these values.