Spry validation with file fields

I understand how to use Spry in DW to validate text fields, checkboxes, radios and so fourth but how do you validate a file field with the same validating features (CSS Styles)? I try just using a textfield and then change it to a file field but Spry gets confused.

Thanks

<script src="SpryAssets/SpryValidationTextField.js" type="text/javascript"></script>
<link href="SpryAssets/SpryValidationTextField.css" rel="stylesheet" type="text/css" />
 
<span id="sprytextfield2">
            <label>
            <input type="file" name="company_logo" id="company_logo" />
            </label>
            <span class="textfieldRequiredMsg">A value is required.</span></span>
 
<script type="text/javascript">
<!--
 
var sprytextfield2 = new Spry.Widget.ValidationTextField("sprytextfield2", "none", {validateOn:["blur"]});
 
//-->
</script>

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