Uploaded file must be opened error when using tempfile plugin

I have a simple file attachment uploader implemented that works great. Storage is GCS. On my model before_save callback I store the size of the image file simply accessing: file.size

When I add the tempfile plugin, I get a Shrine::Error - uploaded file must be opened

Calling file.open prior to file.size works.

Why does this happen? Is this a bug?

Thanks!