I'll make sure that you can successfully upload your file, kwvint.

Currently, QuickBooks Online only supports these file extensions for manually uploading of bank transactions:

• Comma-Separated Values (CSV)
• QuickBooks Online (QBO)
• Quicken (QFX)

It could be that the .qbo file has been damaged that's why QuickBooks was unable to read it. To isolate this issue, we can download them again from your bank's website, then reupload it again to QuickBooks. You can refer to this article to be guided in doing it: Manually upload transactions into QuickBooks Online.

If the same thing happens, we can perform some browser troubleshooting steps to fix this issue. It's possible that the accumulated cache in the system is the cause of that error. First, let's access your QuickBooks account in a private/incognito browsing window. Doing this process will help identify if it's a browser-related issue since this mode doesn't save site data. You can use these shortcut keys:
 

• Google Chrome: Ctrl + Shift + N
• Safari 11 or newer: ⌘ + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P

Then, let's upload the file again. If it succeeds, we'll have to go back to your regular browser then clear the cache to delete the stored junk files and improve the system performance. In addition, we can use other supported browsers to further isolate the issue. 

Once all of your transactions are in QuickBooks, we can now start matching and categorizing them.

I'll be right here to continue helping if you have further questions or any other concerns. Feel free to tag me in your comments.