Convertir XLS en XLSB par programme?

J'écris ce code powershell pour convertir de nombreux *.les fichiers xls dans de nombreux dossiers de manière récursive.
ce script promte pour choisir un dossier , convertir tous les fichiers et supprimer l'original (déplacer vers la corbeille)
l'affichage de chaque nom de fichier dans la console powershell.

<#
    .SYNOPSIS
    Covert all *.xls files recursivly in a provided path
    .DESCRIPTION
    XLS files within a provided path are recursively enumerated and convert to XLSB files (with macro).
    The original XLS files are deleted if newfile has created (in trash), a new XLSb file replace the old file.
    #>
    $autor='alban Lopez'
    $version=0.85
    $email='[email protected]'

    function ConvertTo-XLSB {
    <#
    .SYNOPSIS
    XLS files within a provided path are recursively enumerated and convert to XLSB files.
    .DESCRIPTION
    XLS files within a provided path are recursively enumerated and convert to XLSB files.
    The original XLS files remain intact, a new XLSB file will be created.
    .PARAMETER Path
    This parameter takes the input of the path where the XLS files are located.
    .PARAMETER Visible
    Using the parameter will show you how Excel does the work. Not using the parameter will enable Excel 
    to accomplish its tasks in the background.
    Note: Bu not using this parameter you will be able to convert some XLS files which have corruptions 
    in them, when using the parameter and therefor the Excel GUI will give you an error.
    .PARAMETER ToFolder
    This parameter enables you to provide a location where the file is saved. When this parameter is 
    not used, the file will be saved as an XLS file in the same location as where the 
    original XLS file is located.
    .EXAMPLE
    ConvertTo-XLSB -Path 'D:\Data12'
    .EXAMPLE
    ConvertTo-XLSB -Path 'D:\Data12' -Visible
    .EXAMPLE
    ConvertTo-XLSB -Path 'D:\Data12' -ToFolder 'D:\Data12XLSB'
    .EXAMPLE
    ConvertTo-XLSB -Path 'D:\Data12' -Visible -ToFolder 'D:\Data12XLSB'
#>
    [cmdletbinding()]

        param (
            [parameter(mandatory=$true)][string]$Path,
            [parameter(mandatory=$false)][switch]$Visible,
            [parameter(mandatory=$false)][string]$ToFolder
        )
        begin {
            $Excel = New-Object -ComObject excel.application
            $Excel.DisplayAlerts = $false
            # $xlFixedFormat = [Microsoft.Office.Interop.Excel.XlFileFormat]::xlWorkbookDefault # xlsx
            $xlFixedFormat = [Microsoft.Office.Interop.Excel.XlFileFormat]::xlExcel12 # 50 = xlsb
            $shell = new-object -comobject "Shell.Application"

            $count = 0
            $count_OK = 0
            $count_Nok = 0

            if ($Visible -eq $true) {
                $Excel.visible = $true
            } else {
                $Excel.visible = $false
            }
            $filetype = "*xls"
        } process {
            if (Test-Path -Path $Path) {
                Get-ChildItem -Path $Path -Include '*.xls' -recurse | ForEach-Object {
                    if ($ToFolder -ne '') {
                        $FilePath = Join-Path $ToFolder $_.BaseName
                    } else {
                        $FilePath = ($_.fullname).substring(0, ($_.FullName).lastindexOf("."))
                    }
                    $FilePath += ".xlsb"
                    $WorkBook = $Excel.workbooks.open($_.fullname)
                    $WorkBook.saveas($FilePath, $xlFixedFormat)
                    $WorkBook.close()
                    $OldFolder = $Path.substring(0, $Path.lastIndexOf("\")) + "\old"
                    if (test-path $FilePath){
                        $count_OK++
                        Write-Host -nonewline "$count_OK > "
                        Write-Host $_.fullname -ForegroundColor Cyan
                        $item = $shell.Namespace(0).ParseName("$($_.fullname)")
                        $item.InvokeVerb("delete")
                    } else {
                        $count_Nok++
                        Write-Host -nonewline "$count_Nok > "
                        Write-Host $_.fullname -ForegroundColor red
                    }
                    $count++
                }
            } else {
                return 'No path provided or access has been denied.'
            }
        } end {
            Write-Host '========================================================' -ForegroundColor yellow
            Write-Host -nonewline "Total : $count";
            Write-Host -nonewline " /Erreurs : $count_Nok /" -ForegroundColor red;
            Write-Host "convertis : $count_ok" -ForegroundColor green;
            Write-Host '========================================================' -ForegroundColor yellow

            $Excel.Quit()
            $Excel = $null
            [gc]::collect()
            [gc]::WaitForPendingFinalizers()
        }
    }


#=============================================================================
# Displays a select file dialog box, returning the path to a CSV file.
#=============================================================================
function Read-FolderBrowserDialog([string]$Message, [string]$InitialDirectory)
{
    $app = New-Object -ComObject Shell.Application
    $folder = $app.BrowseForFolder(0, $Message, 0, $InitialDirectory)
    if ($folder) { return $folder.Self.Path } else { return $false }
}

''
'Choisir le dossier source >'
$source = Read-FolderBrowserDialog -Message "Dossier source"

while ($source)
{
    "Convertion de tous les fichiers du dossier : $source"
    $ConvTime = Measure-Command {ConvertTo-XLSB -Path $source}
    Write-Host "$($ConvTime.Hours):$($ConvTime.Minutes):$($ConvTime.Seconds)";
    ''
    "End"
    ''
    'Choisir le dossier source >'
    $source = Read-FolderBrowserDialog -message "Dossier source" -InitialDirectory $source
    #$dest = Select-FolderDialog -message "Dossier Destination (sera conservé)" -RootFolder $source
}
start-sleep -s 30