I've cobbled up a quick powershell script for the purpose when I got my Duet. It watches a folder for new/changed .gcode files and offers to upload them.

# adjust these to your values
$folder = "e:\3dprint\gcodes"
$filter = "*.gcode"
$Global:baseUri = "http://duet3d.local/rr_upload?name=gcodes/"

	
Add-Type -AssemblyName PresentationCore,PresentationFramework
$Global:lastPath = ''
$Global:lastTime = Get-Date

$Watcher = New-Object IO.FileSystemWatcher $folder, $filter -Property @{ 
    IncludeSubdirectories = $false
    NotifyFilter = [IO.NotifyFilters]'FileName, LastWrite'
}

$changeAction = [scriptblock]::Create('
    $path = $Event.SourceEventArgs.FullPath
    $name = $Event.SourceEventArgs.Name
    $timeGenerated = $Event.TimeGenerated

    # usually multile change events are triggered when a file is written, no need to spam those
    if ($Global:lastPath -ne $path -or ($Global:lastTime - $timeGenerated).TotalSeconds -lt -10) {

        $Global:lastPath = $path;
        $Global:lastTime = $timeGenerated
        $msgBoxResult =  [System.Windows.MessageBox]::Show(("Upload `"{0}`" to the printer?" -f $Event.SourceEventArgs.Name),"GCode Upload","YesNo","Question")

        switch  ($msgBoxResult) {
            "Yes" {
                try {
                    Invoke-WebRequest -Uri ("{0}{1}" -f $baseUri, $name) -Method Post -InFile $path 
                } catch {
                }
            }
            "No" {
            }
        }
        }
')

$onChanged = Register-ObjectEvent $Watcher Changed -SourceIdentifier FileChanged -Action $changeAction

# prevent the console from closing
while (1) {
    sleep 2
}

Save somewhere with .ps1 extension and edit line 2-4 to match your settings. To run it rightclick the file -> "Run with PowerShell" and leave the powershell window open. To trigger the upload question just save your gcode file into the folder you specified in line 2.