Hello
What plan did you upgrade to, is it a personal type of plan, or a business plan?
Do you send email from the app, or from Outlook Classic app, or New Outlook app?
Do you run OneDrive Desktop app on the computer?
Can you please describe the steps you take to attach the file? Did you click on the attachment button, then browse this PC option, then go to the OneDrive folder, locate the file and click on it to attach?