How to get the value of a file type input field

In my dialog,there is an element:

{
     type : 'file',
     label : 'choose your file:',
     labelLayout : 'vertical',
     id : 'id2'            
}

Now,I want to get the value of this element,but CKEDITOR.dialog.getCurrent().getValueOf('thisTable','id2') return a empty string.
If it is a HTMLInputElement(<input type="file" name="id2" id="fileid" />), I can get the value by fileid.vlaue.