Parse CSV and decode base64 - regex

Question

The goal of this script is supposed to be to find any base64 encoded strings within a particular row of a CSV, grab only the base64, add the correct amount of padding charcters to it, decode it, and write it to an output.
function Get-FileName($initialDirectory) {
[System.Reflection.Assembly]::LoadWithPartialName("System.Sindows.Forms") | Out-Null
$OpenFileDialog = New-Object System.Windows.Forms.OpenFileDialog
$OpenFileDialog.InitialDirectory = $initialDirectory
$OpenFileDialog.Filter = "CSV (*.csv)| *.csv"
$OpenFileDialog.ShowDialog() | Out-Null
$OpenFileDialog.Filename
}
$inputfile = Get-FileName "C:\My Documents\"
$inputdata = Import-Csv $inputfile
$output = "C:\My Documents\output.csv"
$regex = [regex]::New('(?<=-[Ee]ncoded[Cc]ommand\s"??)[\w\d=\/!]{0,792}')
$csv1 = $inputdata | select -ExpandProperty 'NameOfRow' | ? {$_ -ne ""}
| Select-String -Pattern $regex -Allmatches | % {$_.Matches}
| ? {$_.Value -ne ""} | % {$_.Value}
foreach ($line in $csv1) {
$csvL = $line.Length
$csvM = $csvL %= 4
if ($csvM | % {$_ -eq 1}) {
$line | % {$_ + "==="}
} elseif ($csvM | % {$_ -eq 2}) {
$line | % {$_ + "=="}
} elseif ($csvM | % {$_ -eq 3}) {
$line | % {$_ + "="}
}
$decode = [System.Text.Encoding]::ASCII.GetString([System.Convert]::FromBase64String($line))
$decode > $output
}