When you add Digital File Pro to a page, it creates a group on the bindings panel called "WA Uploaded Files (<fileField>)"
where <fileField> is the name given to the file form field.
In the bindings group is the Server File Name bindings. there is also a Server File name for each upload folder you add. it will be named "<uploadFolder>[FildeName]: Server File Name"
where <uploadFolder> is the folder you specified to upload the image to.
I am including a screen shot of my bindings panel. I created two uploads, one to "large" the other to the folder named "thumb"
In DataAssist, for the large image, I would use the "large[Filename]: Server File Name" binding. For the thumbnail column, I would use the "thumb[Filename]: Server File Name"