Support uploading files

[bongole]

This patch supports uploading files.

To upload file, you should write routes.rb like below:

ApiTaster.routes do
   post '/items', {
        :item => {
           :title => 'test',
           :content => :file  # this generates file upload form.
         }
    }
end

[fredwu]

Very nice! Merge, thanks!