attribute names with [] in a Form Model

In a Form Model, how to give an attribute name with []?

For example, if it is possible to have an attribute with name "file[]", then we can use the following code

$form=$this->beginWidget('CActiveForm');

$form->fileField($contactFormModel,'file[]');

to generate HTML

<input type="file" name="ContactForm[file][]" />

If you need to get such html code:

<input type="file" name="ContactForm[][file]" />

you need to write

echo $form->fileField($contactFormModel,'[]file');