使用 Powershell 过滤文件扩展名

我喜欢搜索一些带有预定义扩展名的旧文件。（如 .doc、.xls、.txt 等）在特定文件夹中。我将扩展名放在一个变量中：

$wildcards = @(".txt",".doc",".xls")

当我在这个变量中放置一个新值时，它必须更改另一个变量。($collection)

这是我的代码：

$folder = Get-ChildItem C:\test\test1\
$date = (Get-Date).Add(-1)
$wildcards = @(".txt",".doc",".xls")


function Get-WildCards { 

$counter = 0;
$counterEnd = $wildcards.Count
$collection = New-Object System.Collections.ArrayList 
$filter = "$" + "_.Extension" + " -like $" + "wildcards[" + $counter + "]}"
$or = " -or "

    for ($counter = 0; $counter -lt $counterEnd; $counter++) {

        $filter = "$" + "_.Extension" + " -like $" + "wildcards[" + $counter + "]}"
        $collection += $filter + $or
    }

$collection = [String]::Join('', $collection)
$collection = $collection.ToString()
$collection = $collection.TrimEnd($or)
} 

Get-WildCards

$folderPath = $folder.FullName

$files = Get-ChildItem -Path $folderPath -File:$true |
 Where-Object {$collection} 

$files

如果我运行此代码，Where-Object 条件将不会以这种方式工作。它不会过滤扩展名。我怎样才能实现它的工作原理？任何帮助表示赞赏。