Dear Sedric,
Greetings!! Good day and thank you for posting this in our forum.
As per your description, you cannot add the "Upload File" option to a newly created form while using Microsoft Forms.
I would like to suggest checking in the form's settings if "Who can fill out this form" option is Anyone can respond?
If yes, "Upload File" option is not available. You may switch to “Only people in my organization can respond” or “Specific people in my organization can respond” and then check if you can use "Upload File" option.
As shown below in the screenshot:
I would also like to recommend checking the official article below:
Add questions that allow for file uploads - Microsoft Support
Please note that my first response may not resolve your current situation immediately. But, with your help and more detailed information, we can work together to find a specific guidance for your current query.
I appreciate your cooperation and valuable time.