The article is a little hard to understand, but it has some pretty
good information about creating additional buttons in libraries so you
can say “New Excel file” or “New Word file” no matter the default
document library's new file type:
http://msd2d.com/newsletter_tip.aspx?section=sharepoint&id=cd3fd32d-b989-4bff-aae7-a65846b795b6