Guest User and internal form

We’re currently facing following scenario:

We want to be able to have external people (outside the tenant) to be able to provide us some information along with files. We tried to accomplish this via a customer voice setup and a file upload. Since the file upload restricts the form to be submitted only by internal people within the tenant, we tried to bypass this with adding the external people as guest users.

However, this does not seem to work and guest users aren't treated as internal users, even with a license included. Is there any other way than creating them with an internal profile?

Thanks