Shared directory with external users, who uploaded that file?

Hi everybody!
Maybe I’m not seeing something, but my situation is this:
One of our users needs to share a directory with an external user, and when I say external I mean not a NextCloud declared user.
He shared the directory, and gives the external user permissions to upload files.
What he needs to understand is how can he know that a specific file was uploaded by him, or by the external user, because he didn’t share it with ony one external user, but three different users. So, each can upload its files, but I don’t see where he can check which file was uploaded by who.
Is there any feature, app, or place where to check for this?
Thanks a lot in advance for your time and attention!
Best regards,