fixup! erp5_interface_post: action to export all exportable Letter Post in only one PDF document

bt5/erp5_interface_post/SkinTemplateItem/portal_skins/erp5_interface_post/LetterPostModule_exportExportableLetterPostActivity.py

@@ -18,6 +18,8 @@ result_pdf = portal.document_module.newContent(
   portal_type='PDF',
   title='Impression of Letter Post',
   data=merged_pdf_content,
+  content_type='application/pdf',
+  filename='letter_export.pdf',
 )
 
 portal.portal_notifications.activate(activity='SQLQueue').sendMessage(