gistfile1.js

var file = 'path/to/a/file',
    stats = fs.statSync(file);

var bufferSize = 4 * 1024 * 1024;

options = {
   from: 0,
   to: bufferSize,
   bufferSize: bufferSize
}

function flush(options) {
   fs.createReadStream(file, options)
      .on('data', function(buffer) {
         res.write(buffer);
      });
}

req.connection.on('drain', function() {
   options.from += bufferSize;
   options.to   += bufferSize;

   if (options.from > stats.size) {
       res.end();
       return;
   }

   if (options.to > stats.size) {
       options.to = undefined;
       options.bufferSize = (stats.size - options.from);
   }

   flush(options);
});