Anonymous Users cannot open XLSX files from a SharePoint document library

$siteUrl = "http://URL_of_your_SITE";
$site = New-Object Microsoft.SharePoint.SPSite($siteurl);
$web = $site.OpenWeb();
 
$enumPerms = [Microsoft.SharePoint.SPBasePermissions];
 
Write-Host $web.AnonymousPermMask64;
$web.AnonymousPermMask64 = $web.AnonymousPermMask64 -bor $enumPerms::OpenItems
$web.Update();
Write-Host $web.AnonymousPermMask64;
 
$web.Dispose();
$site.Dispose();

http://support.microsoft.com/kb/2498047