What i meant by additional Bindings is that once you apply Digital File Pro, you get additional Bindings that could help you determine if the upload file was selected or if the upload was successful.
I have attached a page that demonstrate how to use the Status Code bindings with Server Side Validation to ensure a file was selected. the tricky part here is make sure the Server Side Validation is listed below the Digital File Pro Code.
In regard to checking new user name, i recommend that you follow this steps to accomplish that:
1. create a record set based on the username when the form is submitted
2. update the insert server behavior to only insert if the number of rows return from the record set is 0
3. you can then use the Show IF Server Behavior in Dreamweaver to display a message if the number of rows > 0 stating that the username already exists