Packer doesn't seem to upload files to Windows

I’m trying to learn Packer for Windows after using it with Linux for about a year. I don’t know Windows well; but I can’t seem to upload files with Packer to my Windows instance. The file says it uploads, but when I start the instance; the file is not there. What am I doing wrong?

poc.json:

{
    "variables": {
        "environment": "redacted",
        "contact": "redacted",
        "service": "redacted"
    },
    "builders": [
        {
            "profile": "nonprod",
            "region": "us-west-2",
            "type": "amazon-ebs",
            "subnet_id": "redacted",
            "security_group_id": "redacted",
            "source_ami": "redacted",
            "instance_type": "t2.micro",
            "communicator": "winrm",
            "winrm_username": "Administrator",
            "ami_name": "{{user `service`}}-v7.0-{{user `environment`}}-Packer-{{timestamp}}",
            "tags": {
                "Name": "{{user `service`}}-v7.0-{{user `environment`}}-Packer-{{timestamp}}",
                "Service": "{{user `service`}}",
                "Environment": "{{user `environment`}}",
                "Contact": "{{user `contact`}}",
                "Packer": "true"
            },
            "run_tags": {
                "Name": "{{user `service`}} Packer Builder",
                "Service": "{{user `service`}}",
                "Environment": "{{user `environment`}}",
                "Contact": "{{user `contact`}}",
                "Packer": "true"
            },
            "launch_block_device_mappings": [
                {
                    "device_name": "xvdb",
                    "volume_size": 500,
                    "volume_type": "gp2",
                    "delete_on_termination": true
                }
            ]
        }
    ],
    "provisioners": [
        {
            "type": "powershell",
            "inline": [
                "C:/ProgramData/Amazon/EC2-Windows/Launch/Scripts/InitializeInstance.ps1 -Schedule",
                "C:/ProgramData/Amazon/EC2-Windows/Launch/Scripts/SysprepInstance.ps1 -NoShutdown",
                "Get-Disk | where partitionstyle -eq \"raw\" | Initialize-Disk -PartitionStyle GPT -PassThru | New-Partition -AssignDriveLetter -UseMaximumSize | Format-Volume -FileSystem NTFS -NewFileSystemLabel \"Data Drive\" -Confirm:$false",
                "mkdir C:/Packer/",
                "mkdir C:/Packer/Misc_Installers/"
            ]
        },
        {
            "type": "file",
            "source": "./files/startup.ps1",
            "destination": "C:\\Packer\\"

        }
    ]
}

PACKER_LOG=1 packer build poc.json:

2020/05/20 08:04:05 [INFO] (telemetry) Starting provisioner file
==> amazon-ebs: Uploading ./files/startup.ps1 => C:\Packer\
2020/05/20 08:04:05 packer-builder-amazon-ebs plugin: Uploading file to 'C:\Packer\startup.ps1'
2020/05/20 08:04:05 packer-provisioner-file plugin: [INFO] 1515 bytes written for 'uploadData'
2020/05/20 08:04:05 [INFO] 1515 bytes written for 'uploadData'
startup.ps1 1.48 KiB / 1.48 KiB [=====================================================================================================] 100.00%
startup.ps1 1.48 KiB / 1.48 KiB [=====================================================================================================] 100.00%
2020/05/20 08:04:08 packer-builder-amazon-ebs plugin: <Objs Version="1.1.0.1" xmlns="http://schemas.microsoft.com/powershell/2004/04"><Obj S="progress" RefId="0"><TN RefId="0"><T>System.Management.Automation.PSCustomObject</T><T>System.Object</T></TN><MS><I64 N="SourceId">1</I64><PR N="Record"><AV>Preparing modules for first use.</AV><AI>0</AI><Nil /><PI>-1</PI><PC>-1</PC><T>Completed</T><SR>-1</SR><SD> </SD></PR></MS></Obj></Objs>#< CLIXML
2020/05/20 08:04:08 packer-provisioner-file plugin: closing
2020/05/20 08:04:08 closing
2020/05/20 08:04:08 packer-builder-amazon-ebs plugin: <Objs Version="1.1.0.1" xmlns="http://schemas.microsoft.com/powershell/2004/04"><Obj S="progress" RefId="0"><TN RefId="0"><T>System.Management.Automation.PSCustomObject</T><T>System.Object</T></TN><MS><I64 N="SourceId">1</I64><PR N="Record"><AV>Preparing modules for first use.</AV><AI>0</AI><Nil /><PI>-1</PI><PC>-1</PC><T>Completed</T><SR>-1</SR><SD> </SD></PR></MS></Obj><startup.ps1 1.48 KiB / 1.48 KiB [==================================================================================================] 100.00% 2s
2020/05/20 08:04:08 [INFO] (telemetry) ending file

Just an update on this. The reason I am unable to upload files is because it’s blocked by corporate ‘hardened’ Windows.

I got around the problem by uploading my (6GB) files to S3 and downloading them to the Builder. The pros is that it is MUCH faster than using WinRM.