$extensions = (".bak", ".sfap0", ".sfk")

$fsw = New-Object System.IO.FileSystemWatcher
$fsw.Path = 'd:\test' #mappe som skal overvåges
$fsw.Filter = '*.*'
$fsw.IncludeSubdirectories = $true #overvåg undermapper
$fsw.EnableRaisingEvents = $false

while($true)
{
    $result = $fsw.WaitForChanged([System.IO.WatcherChangeTypes]::Created, 1000)
    if($result.TimedOut){ continue }

    $ext = [System.IO.Path]::GetExtension($result.Name)
    if($extensions -contains $ext)
    {
        $file = Get-Item (Join-Path $fsw.Path $result.Name)
        $file.Attributes = 'Hidden' #skjul fil
    }
}