Second, create a merb app in your rails dir:

--- ruby
    merb-gen app uploader
    cd uploader

    run Merb::Rack::Application.new

require 'cgi'

class File
  def to_s
    path
  end
end

# build a new handler to handler rack's request
  class Uploader 
    def call(env)
      # leverage merb's utility to parse the request.
      # Merb will save the file to a tempfile and save the tempfile's path in request's param
      request = Merb::Request.new(env)

      params = request.params

      # pass the params directly to the real (rails) app
      result = post("http://someplace.com/api", hash_to_params(params)).split("\n")[-1]
     
      # processing result or just ignore it ...
    end

    private

    def post(url, params="")
      curl_cmd = "curl -H \"Content-type: application/x-www-form-urlencoded\" #{url} -d \"#{params}\""
      puts "curl_cmd = #{curl_cmd}"
      f = IO.popen(curl_cmd +" 2>&1")
      result = f.read
      f.close
      result
    end

    def hash_to_params(hash)
      hash.map do |k, v|
        if v.kind_of? Hash
          h = {}
          v.each { |kk, vv| h["#{k}[#{kk}]"] = vv }
          hash_to_params h
        else
          "#{k}=#{CGI.escape v.to_s}"
        end
      end.join("&")
    end

  end

run Uploader.new