Validation errors and file uploads

I've got a form that contains a number of textfields, as well as a
file upload element.

I've noticed that this field doesn't retain it's data after validation
errors. While I know html/http doesn't allow form fields to be pre-
populated per se, it can be faked by stuffing it into the session.

Is there any accepted pattern/best practice for faking it that doesn't
require the file to be tacked to the session?

I should say that I'm using rails 1.2.6, and this is self-rolled file
code in the controller and model.