Powershell job unable to find System.windows.forms

Question

I want to run a background job in my PowerShell-Script to read all file icons from a specific folder into a [System.Windows.Forms.ImageList].

$JobScript = {
    param([String] $Path)
    Add-Type -AssemblyName System.Windows.Forms
    Add-Type -AssemblyName System.Drawing
    
    $Files = [System.IO.DirectoryInfo]::new($Path).GetFiles()
    [System.Windows.Forms.ImageList] $ImgList = [System.Windows.Forms.ImageList]::new()
    $Files.ForEach({
        $Extension = [System.IO.FileInfo]::new($_.FullName).Extension
        if ($Extension -notin $ImgList.Images.Keys)
        {
            $ImgList.Images.Add($Extension, [System.Drawing.Icon]::ExtractAssociatedIcon($_.FullName))
        }
        
    })
    $ImgList
}

$Job = Start-Job -ScriptBlock $JobScript -ArgumentList "C:\Windows"
#Do other stuff
Wait-Job $Job
Receive-Job $Job

The code generates plenty of errors, stating that neither [System.Drawing.Icon] nor [System.Windows.Forms.ImageList] can be found. When I run the code without a job it works just fine. Strangely the System.IO-namespace is working alright.

I guess I need to use add-type inside the script block, but how do I add these system types with it?

EDIT: I updated the code with the help of Mathias' comment.