Your code basically becomes similar to this:

sentinel = object()

class FileObject:
    _uploaded_by: str = None
    uploaded_by = None

    def __init__(self, uploaded_by=sentinel):
        if uploaded_by is sentinel:
            self.uploaded_by = FileObject.uploaded_by
        else:
            self.uploaded_by = uploaded_by

    def save(self):
        print(self.uploaded_by)

    @property
    def uploaded_by(self):
        return self._uploaded_by

    @uploaded_by.setter
    def uploaded_by(self, uploaded_by):
        print('Setter Called with Value ', uploaded_by)
        self._uploaded_by = uploaded_by

Which has the same problem. I'll have to give it some thought.