powershell, concatenate text files in a directory

$invocation = (Get-Variable MyInvocation).Value
$directorypath = Split-Path $invocation.MyCommand.Path
$files = $directoryPath + '\*'
$outFile = $directoryPath + '\AllTextInOneFile.txt'
ls $files -include *.txt, *.otherExtension | gc | out-file $outFile