The wizard in Data Bridge 1.0.6 should be doing this for you already.
I have jsut tested this and the wizard is correctly setting the default file setting. It is using the file column from the Recordset on the update page, but this should be working as expected.
<?php
// WA_UploadResult1 Params Start
$WA_UploadResult1_Params = array();
// WA_UploadResult1_1 Start
$WA_UploadResult1_Params["WA_UploadResult1_1"] = array(
'UploadFolder' => "",
'FileName' => "[FileName]",
'DefaultFileName' => "".($row_WADAps3_products["ProductThumb"]) ."",
'ResizeType' => "0",
'ResizeWidth' => "0",
'ResizeHeight' => "0",
'ResizeFillColor' => "" );
// WA_UploadResult1_1 End
// WA_UploadResult1 Params End
?>