multipart.rb

require 'sinatra'

post '/upload' do
  p params
  unless params[:test] &&
         (tmpfile = params[:test][:tempfile]) &&
         (name = params[:test][:filename])
    @error = "No file selected"
    return 'error'
  end
  STDERR.puts "Uploading file, original name #{ name.inspect}"
  while blk = tmpfile.read(65536)
    # here you would write it to its final location
    STDERR.puts blk.inspect
  end
  "Upload complete"
end

set :port, 8189

$ gem install sinatra
$ ruby multipart.rb -o 0.0.0.0