Upload file - make private before a thing exisits

I have a publicly accessible form, which allows the user to upload a file.
But I need the file to be attached to a thing that is not yet created.
The thing will be created after the upload has happened.

Any technique to make the file private and attach it to the thing being created still?

Thanks!

I think you do want to create a thing at the time of upload in this case.

If right now you are attaching a file directly to a Post, maybe instead you should be attaching it to an Attachment, and each Attachment has a field with the the Post.

Or set up some workflow where the files are tied to the User until you are ready to tie it to the Post, then somehow reupload it again as private and delete the original one, but that sounds clunky.