To upload videos larger than 8MB on discord, you can upgrade to Nitro which is a Discord booter that allows adding files up to 50MB. But if you are not willing to go for the upgrade then there is a workaround using Imagur that will help you to bypass the 8MB limit on Discord.
Imagur is an online image hosting and sharing site where you can also post videos up to 200MB. So, you can upload your videos to Imagur, and then its link can be pasted to the Discord chat.