You can't. It's a limitation of the web browser. For security purposes the value of a file input cannot be set progammatically.
You could sort of work around it by uploading the attachment anyway and then re-associating it with the form, but it would be quite an involved modification and potential security risk, as users would be actually storing files on your server without having to provide a valid CAPTCHA response.
Unfortunately, you can't just store the user-supplied path for the attached file and then re-populate the file upload field.
