You are correct. The "File upload" option in Microsoft Forms is only available when the form is set to "Only people in my organization can respond" or "Specific people in my organization can respond". When the setting is "Anyone can respond," the "Upload File" option is greyed out.

This limitation is due to security and privacy concerns, as allowing anonymous users to upload files could pose a risk. Unfortunately, there isn't a built-in way to enable file uploads for external users in Microsoft Forms at the moment.

If you need external users to upload files, you might consider alternative solutions such as:

• Using a third-party form service that supports file uploads from external users.
• Creating a shared folder in OneDrive or SharePoint where external users can upload files, and then providing the link to that folder in your form.