como puedo asignar el valor a un <input type=file>

[AlZuwaga]

no se puede, por cuestiones de seguridad:

Why can't I pre-populate INPUT TYPE=FILE element?
When uploading a file, users are constantly asking if they can programmatically populate the path in an INPUT TYPE=FILE element. The fact is, users must input this file themselves by typing it in manually or using the provided Browse... button. Otherwise, it would be fairly trivial for malicious users to steal files from users' hard drives...