Dear soy2197192,
Welcome to the Microsoft Community.
I understand that when you upload a file using Copilot you are prompted that the file has exceeded the limit, I understand your frustration and will do my best to help you troubleshoot.
In order to help you better, I'd like to know a little more about it:
- Are you using Copilot on the web or in Office? Like in the picture below or in Office?
According to your description, I found an official article about uploading files to Copilot, you can check if there is any restriction you don't meet: Upload files as a knowledge source - Microsoft Copilot Studio | Microsoft Learn

If you are using it in the web version, you can try to clear your browser cache or try to upload files in privacy mode.
Sincerely hope you can understand that some troubleshooting needs to be done as the initial information may not give us a clear idea of the direction of the problem. You are welcome to contact us at any time if there is any progress.
Thank you for your understanding and patience.
Best regards
Isabella-MSFT | Microsoft Community Support Specialist