I'd also get frustrated if I encountered the same error you're experiencing right now.
I’ll provide assistance with your query, so you'll be able to upload a vendor invoice in QuickBooks Online.
Issue like this occurs when we encounter a browser-related issue. We can fix this by running some basic troubleshooting steps.
I suggest performing the steps through a private window. It's the best place to check for browser-related issues while working with QuickBooks. To save you time, use either of these keyboard shortcuts:
Also, here’s a handy resource you can visit to learn more about the special characters you can add to an attachment’s file name: Acceptable characters in QuickBooks Online.
Keep me posted if you still have questions or concerns about attachments or files in QBO. I'll be around for you. Enjoy the rest of the day and keep safe.