I am stump need to remove square brackets

[dnmvisser (Dick Visser)]

The Win_powershell output variable is a list, and in your case it contains one item: the file content.
This is very clearly stated in the module docs:

https://docs.ansible.com/ansible/latest/collections/ansible/windows/win_powershell_module.html#return-output

So just pick the first item:

set_fact:
uuid: "{{ file_contents.output[0] }}