encode_multipart_form_data

# File lib/net/http.rb, line 1985
    def encode_multipart_form_data(out, params, opt)
      charset = opt[:charset]
      boundary = opt[:boundary]
      require 'securerandom' unless defined?(SecureRandom)
      boundary ||= SecureRandom.urlsafe_base64(40)
      chunked_p = chunked?

      buf = ''
      params.each do |key, value, h={}|
        key = quote_string(key, charset)
        filename =
          h.key?(:filename) ? h[:filename] :
          value.respond_to?(:to_path) ? File.basename(value.to_path) :
          nil

        buf << "--#{boundary}\r\n"
        if filename
          filename = quote_string(filename, charset)
          type = h[:content_type] || 'application/octet-stream'
          buf << "Content-Disposition: form-data; "              "name=\"#{key}\"; filename=\"#{filename}\"\r\n"              "Content-Type: #{type}\r\n\r\n"
          if !out.respond_to?(:write) || !value.respond_to?(:read)
            # if +out+ is not an IO or +value+ is not an IO
            buf << (value.respond_to?(:read) ? value.read : value)
          elsif value.respond_to?(:size) && chunked_p
            # if +out+ is an IO and +value+ is a File, use IO.copy_stream
            flush_buffer(out, buf, chunked_p)
            out << "%x\r\n" % value.size if chunked_p
            IO.copy_stream(value, out)
            out << "\r\n" if chunked_p
          else
            # +out+ is an IO, and +value+ is not a File but an IO
            flush_buffer(out, buf, chunked_p)
            1 while flush_buffer(out, value.read(4096), chunked_p)
          end
        else
          # non-file field:
          #   HTML5 says, "The parts of the generated multipart/form-data
          #   resource that correspond to non-file fields must not have a
          #   Content-Type header specified."
          buf << "Content-Disposition: form-data; name=\"#{key}\"\r\n\r\n"
          buf << (value.respond_to?(:read) ? value.read : value)
        end
        buf << "\r\n"
      end
      buf << "--#{boundary}--\r\n"
      flush_buffer(out, buf, chunked_p)
      out << "0\r\n\r\n" if chunked_p
    end