Get Filename In Process model

On file upload I want to change [Document:1234] to fileName.png.
Here is what I have currently

I've tried various expression in

But whatever I do, I don't get the filename saved in a variable.
Things tried include (but I may have done them wrong):

• using a a!map
• using the document(documentId,"name")
  • however [Document:1234] is not a number
  • using a!replace(pv!file,"Document:",""), and that did not work

I simply need to get the Document Id value so that I can use the document() function.
I feel that I'm running out of things to try.

Here is my input