mello.rb

require 'net/http'

uri = URI.parse("https://mydomain.com/attachments")
request = Net::HTTP::Post.new(uri)
request["Authorization"] = "Got this"

req_options = {
  use_ssl: uri.scheme == "https",
}

form_data = [['readme', File.open('README.md')]]

request.set_form form_data, 'multipart/form-data'

response = Net::HTTP.start(uri.hostname, uri.port, req_options) do |http|
  http.request(request)
end