Multi upload file and validation form

[eluser]loca[/eluser]
I have a form with multi file upload, each file field I have a button to choose a file.
I use form validation but I have problem:
Assume that I have some file fields in view file

Code:

<input name="file1" type="file" />
<input name="file2" type="file" />

In my controller, to validate these file field I use structure of code as in Codeigniter User Guide following

Code:

$this->form_validation->set_rules('username', 'Username', 'callback_username_check');

when I select a file for "file1" and not select any file for "file2". In this time, I don't want to upload any file to my server if all field is required. But if use callback function. Each times I check for validate. "File1" will be uploaded to server.