You have to set plugable paraser for text/plain to html parser
here's hack (you'll have to change get to get_html):

class WWW::Mechanize
   def get_html(url)
     old_parser= @pluggable_parser['text/plain']
     @pluggable_parser['text/plain']=@pluggable_parser['text/html']
     bdy = get(url)
     pluggable_parser['text/plain']=old_parser
     bdy
   end
end

and other way around (always get file for html)

class WWW::Mechanize
   def get_file(url)
     old_parser= @pluggable_parser['text/html']
     @pluggable_parser['text/html']=::WWW::Mechanize::File
     bdy = get(url).body
     pluggable_parser['text/html']=old_parser
     bdy
   end
end