You can't set the value of a file field in html for security reasons so that you can't take files from someone from a hidden file field on a form, so that technique won't work.
What you can do, is populate an image field above or beside your file field, or put the value to the left or right of the file field.
Then in the file upload server behaviors (you would use one per upload field), you would specify the default value of the upload to be equal to the field in the database associated with that photo.
The input file field stays blank, but they can see if there is an image or what the image is by adding a thumbnail of the image beside it. Then if they don't specify a value, it stays blank, and dataAssist will use the default value for the image, which is the original value from the updated recordset (what you currently are trying to set to the value of the file field, should be the default value of the upload server behavior).