您可以从以下内容开始：

add_action( \'comment_form_logged_in_after\', \'wpse_102612_additional_fields\' );// only for logged users

function wpse_102612_additional_fields () {
echo \'<p class="comment-form-upload">\'.
\'<label for="upload">\' . __( \'Upload your file\' ) . \'</label>\'.
\'<input id="upload" name="upload" type="file" size="30" /></p>\';

}