Net::HTTPGenericRequest#encode_multipart_form_data

# File tmp/rubies/ruby-3.3.0/lib/net/http/generic_request.rb, line 312
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