Sums the sizes of ".bak", ".diff", and ".trn" files in a folder

Get-BackupFilesSize.ps1

Function Get-BackupFilesSizes {
    $Bak = $Diff = $Trn = 0;

    $Bak = (Get-ChildItem -File | 
            Where-Object { $_.Extension -eq '.bak' } | 
            Measure-Object -Property Length -Sum).Sum;
    $Diff = (Get-ChildItem -File |
            Where-Object { $_.Extension -eq '.diff' } |
            Measure-Object -Property Length -Sum).Sum;
    $Trn = (Get-ChildItem -File |
            Where-Object { $_.Extension -eq '.trn' } |
            Measure-Object -Property Length -Sum).Sum;

    [PSCustomObject]@{
        'Bak Files (GB)'  = $Bak / 1GB
        'Diff Files (GB)' = $Diff / 1GB
        'Trn Files (GB)'  = $Trn / 1GB
    };
};