got this from one of the job themes,
I just added it in the form section on the item-sidebar.php
<div class="control-group"><?php if( osc_item_attachment() ) { ?>
<label for="subject"><?php _e('Your file', 'bender') ; ?></label> <?php ContactForm::your_attachment() ; ?>
<?php } ?></div>
also add this to the "form action" string:
enctype='multipart/form-data'