ConvertTo-Base64

function ConvertTo-Base64
{
    param
    (
        [string] $SourceFilePath,
        [string] $TargetFilePath
    )

    function Resolve-PathSafe
{
    param
    (
        [string] $Path
    )

    $ExecutionContext.SessionState.Path.GetUnresolvedProviderPathFromPSPath($Path)
}

    $SourceFilePath = Resolve-PathSafe $SourceFilePath
    $TargetFilePath = Resolve-PathSafe $TargetFilePath

    $bufferSize = 9000 # should be a multiplier of 3
    $buffer = New-Object byte[] $bufferSize

    $reader = [System.IO.File]::OpenRead($SourceFilePath)
    $writer = [System.IO.File]::CreateText($TargetFilePath)

    $bytesRead = 0
    do
    {
        $bytesRead = $reader.Read($buffer, 0, $bufferSize);
        $writer.Write([Convert]::ToBase64String($buffer, 0, $bytesRead));
    } while ($bytesRead -eq $bufferSize);

    $reader.Dispose()
    $writer.Dispose()
}