How to write a test sequence for JSON body?

I have this 'POST' json body

{
    "guid": "",
    "objectId": "",
    "systemCode": "",
    "callbackURL": "",
    "packageType": "",
    "files":
            [
                {
                    "fileName": "file01.pdf",
                    "documentType": "example"   
                }
            ]
}

I want to create a sequence in "Pre-req." or "Tests" for the "fileName" object so it'll automatically change the fileName within the body. For example, I have two files "file01.pdf" and "file02.pdf". Is there an option for automatic name changing after I ran the post case - it'll switch from 'file01.pdf' to 'file02.pdf' ?

Thank you in advance