multipart.js

var http = require('http'),
    couchdb = http.createClient(5984, 'localhost'),
    uuid = require('node-uuid');

var id = uuid().replace(/-/g, '');
var request = couchdb.request('PUT', '/test/'+id +'?batch=ok', {
  "host": "localhost",
  "Content-Type": "multipart/related;boundary=\"frontier\""
});

var content = new Buffer("kolor:\r\nŻółty");

var doc = {foo: "bar"};
doc._attachments = {
  'test.txt': {
    'follows': true,
    'content_type': 'text/plain',
    'length': content.length
  }
}

request.end(
  "--frontier\r\n" +
  "content-type: application/json\r\n" +
  "\r\n" +
  JSON.stringify(doc) +
  "\r\n--frontier\r\n" +
  "\r\n" +
  content +
  "\r\n--frontier--"
);

request.on('response', function (response) {
  console.log('STATUS: ' + response.statusCode);
  console.log('HEADERS: ' + JSON.stringify(response.headers));
  response.setEncoding('utf8');
  response.on('data', function (chunk) {
    console.log('BODY: ' + chunk);
  });
});