multipart-post-sinatra.rb

require 'sinatra'
require 'net/http/post/multipart'

get '/' do
  file = Tempfile.new("temp.txt")
  file << "test contents"
  file.flush

  request = Net::HTTP::Post::Multipart.new(
    "http://localhost:4567/upload",
    'file' => UploadIO.convert!(
      file,
      'application/octet-stream',
      "temp.txt",
      file.path))

      response = Net::HTTP.start("localhost", 4567) { |http| http.request(request) }
      return response.body
end

post '/upload' do
  p params
end