get_attachment_template needs namespace

[ptahdunbar]

For consistency with the other template types, attachment based templates should have 'attachment' prepended to the template files.

Also fixed the order it looks for mime types from most specific to more generic:

• attachment-{$type[0]}_{$type[1]}.php
  
• attachment-{$type[1]}.php
  
• attachment-{$type[0]}.php
  
• attachment.php
  

[nacin]

This isn't the least backwards compatible.

It's probably best to leave it alone. I referenced filesystem namespace clutter in #12105 but I just think we shouldn't introduce more namespace collision issues -- the current ones we can live with.