doing it that way would require hand coding to accomplish
the file would need to be uploaded to the server. thats what happens with the attachment section of this forum, it is a separate form that uploads files and saves the file name to a session variable.
If you need help setting this up, we could help in a premiere support appointment:
as for the validation, you would use server validation and the validation show if behavior to show an error if validation fails. see this tutorial for details on using server validation: