I could not find Thumbs[FileName]: Server File Name from the image when I double click on Insert Record from Server Behaviours (Page 2 of options > then clicked the thumb column and lightening bolt > WA Upload files - it only lists Server File Name for the main image not the thumb.
I have worked around it for now making it call the thumbnail version the same as the main image eg [filename] instead of [filename]_thumb, but it places it in a different directory. Need to work on the update page now so not sure how this will work with your instructions as it mentions the hidden field etc??