When using a Nintex Forms as start event of a workflow, if you use a File Upload control actually you don't have a way to get a link to the file that could be used later in tasks. This is a different behaviour from File Upload control in task form where you can retrieve a link to the file,
Please make the output for the File Upload control in start form equal to the output of the same control in Task form.
This all goes back to using files as objects or file variables that can be displayed on task forms or attached to emails. Please merge this idea with https://ideas.nintex.com/ideas/IDEA-I-2881