inserting image file name and server file name into db

I have created the admin pages using DataAssist and set up an image upload from the insert record page using DFP2. I followed the instructions in the help pdf dfp_da_insert_update_ht.pdf However the instructions do not give full details of how/what to put in the server behavior field under the column for my image thumbnail field in the database. Setting the value to <?php echo $WA_DFP_UploadStatus["WA_UploadResult1"]["serverFileName"]; ?> works great for the main image, however as I have set up the upload to work from one upload automatically creating the thumb image (renaming the file to picname_thumb.jpg) I don't know what to enter for the value of the server behavior to make it add the _thumb bit which is automatically added.

So to summarize the pic is uploading correctly and automatically creatiing a _thumb version from the one upload, and the file name is being inserted into the database main pic field correctly however I don't know how to get it to add the _thumb filename into the thumb filename field of the database