I'm not sure if I completely understand what you're asking, but I can give you some pointers on setting up an upload on an update. If you're setting up the upload and want the update to keep the original if the file field is left blank, you will have to create a recordset that gets the row with the previous file name. Then open up the upload server behavior and click on the lightning bolt next to the default file field. Then select the recordset value for the filename. To ensure that the value is saved, try clicking inside the default file field and hitting tab. Then apply the upload.