gistfile1.txt

var util = require('util')
  , fs = require('fs')
  , stream = require('stream')
  , mmm = require('mmmagic')
  , Magic = mmm.Magic;

var mimeChecker = new stream.Transform();
mimeChecker.data = [];
mimeChecker.mimeFound = false;
mimeChecker._transform = function (chunk, encoding, done) {
  var self = this;
  if (this.data.length < 10) {
    this.data.push(chunk);
    done();
  }
  else if (this.data.length === 10 && !this.mimeFound) {
    var buffered = Buffer.concat(this.data);
    new Magic(mmm.MAGIC_MIME_TYPE).detect(buffered, function(err, result) {
      if (err) return self.emit('error', err);
      if (result === 'text/plain') {
        return self.emit('error', new Error('Wrong MIME type'));
      } else {
        self.data.map(self.push.bind(self)); // re-emit all buffered data!
        self.mimeFound = true;
      }
      done();
    });
  }
  else {
    this.push(chunk);
    done();
  }
};


var inputFile = fs.createReadStream('somefile.nottxt');

var checkedFile = mimeChecker;

checkedFile.on('error', function (err) {
  console.log(err);
});

var outputFile = fs.createWriteStream('someotherfile.nottxt');

inputFile.pipe(checkedFile).pipe(outputFile);