Does anyone know why I can’t upload a file to my invoice, previously there has never been an issue

Thanks for posting here and allowing us to help you with your activity in QuickBooks, @fergyboy30,

It is always our main priority to ensure your issue is addressed immediately. With that being said, I need a few information about the difficulty you're having with our software.

May I know if you're referring to the attachments feature in QBO? If so, what error message are you getting? Also, are you logged in to the QBO mobile app or on a regular browser? Any information will help me check for ongoing issues and reported cases related to your account.

In the meantime, try opening your QuickBooks in a private window. Unexpected behaviors in the program, are sometimes affected by issues in the web browser. This is because the cache will overwrite itself and will not delete any data over time. 

Private browsing prevents cookies from being stored on your computer, making it a great place to identify browser issues. Use these keyboard shortcuts:

Google Chrome: press Ctrl + Shift + N  
Mozilla Firefox: press Ctrl + Shift + P
Safari: press Command + Shift + N

If it rectifies the problem, clear the browser's cache. Clearing the history or log of sites resolves most issues with the web pages.

Keep me updated with the result, as I want to make sure this is taken care of. I'm also here if you have any questions or need help with other QuickBooks processes. Have a productive week!