$strBase     = "http://url/download/"
$strFileBase = "filename"
$aExtensions = ".exe",".aaa",".bbb",".ccc"

foreach ($ext in $aExtensions)
{
   try
   {
    $a = Invoke-WebRequest -Uri $($strBase + $strFileBase + $ext) -OutFile $($strFileBase + $ext)
    write-host  $strBase$strFileBase$ext "downloaded to" $strFileBase$ext
   }
   catch
   {
    $strBase + $strFileBase + $ext + " doesn't exist on server."
   }
}