Fix for Internet Explorer file downloading/export

Still debugging and diagnosing. I haven't had a chance to test this with SSL yet, which has its own set of problems, but this corrected export issues that I was seeing with IE7 and IE8.

In views_export_xls.module, line ~37, remove the drupal_add_http_header line and add these replacement drupal_add_http_header calls beneath $filename:

/**
 * Preprocess xls output template.
 */
function template_preprocess_views_export_xls_view_xls(&$vars) {
  $view     = $vars['view'];
  $fields   = &$view->field;


  $filename = strtr(
    $vars['options']['filename'],
    array('%view' => check_plain($view->name))
  );


  drupal_add_http_header("Cache-Control", "private,pre-check=0,post-check=0,max-age=0");
  drupal_add_http_header("Content-Description", "File Transfer");
  drupal_add_http_header("Content-Disposition", "attachment; filename=$filename");
  drupal_add_http_header("Content-Type", "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet");
  drupal_add_http_header("Content-Transfer-Encoding", "binary");

  // ...

I believe that is the correct Cache-Control line to allow IE to download XLS files over SSL.

The Content-Type should be the newer Open format (XML) vs the legacy XLS format.

In all versions of IE, this should give you the Open/Save dialog correctly. Content-Description is probably not necessary?