When working in Microsoft Planner from within Microsoft Teams, there is no option to upload a file (from your local pc) to a task. While this is very confusing for end-users, Microsoft explains you need to upload the file in Teams and then attach it with the SharePoint option.
But Office 365 wouldn’t be Office 365 if there wasn’t an easy workaround. Simply open your plan in the browser from within Teams or from the app launcher.
Click on the task where you want to upload attachments, and… Magic, the button appears again!
There is a suggestion on the User Voice to have it fixed/changed, go give it a thumbs up if this confuses you too. 🙂