viewxls macro in the macro browser does not work correctly when inserting attachments from a comment on a blog

To reproduce:

• Add a blog post, with an attached xls file
• Comment on the blog post, and use the MB to enter the viewfile macro

The Macro Browser does not default to the current Blog Post, so the smart field dropdowns are empty. Entering the Blog title manually will then add invalid markup with 'page=undefined'.

Created as part of CONF-16935