I searched for content-type xslx and found numerous posts

HCL Notes/Domino 8.5 Forum (includes Notes Traveler)

Subject:

http://stackoverflow.com/questions/2937465/what-is-correct-content-type-for-excel-files

So I believe you'd need:

Print |Content-Type:application/vnd.openxmlformats-officedocument.spreadsheetml.sheet|

Then reading further it sounds like that will throw an warning in excel, that the only way to remove is a registry edit. If you want to created a real XLSX file, then you'll need to use the Office XML format I believe.

insert this key in your registry to suppress the notification:
[HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Excel\Security] “ExtensionHardening”=dword:00000000
You can accomplish the above by doing the following:
1. Open your Registry (Start -> Run -> regedit.exe)
2. Navigate to HKEY_CURRENT_USER\SOFTWARE\MICROSOFT\OFFICE\12.0\EXCEL\SECURITY
3. Right click in the right window and choose New -> DWORD
4. Type “ExtensionHardening” as the name (without the quotes)
5. Verify that the data has the value “0″

Feedback response number CTYR9Y7SEW created by ~Mark Bubnitexader on 07/07/2015