Node MIME-Type checker using mmmagic. This can be used as an inspiration for more sophisticated solutions.

mimeChecker.js

// Copyright (c) 2013 Christian Maniewski. License: MIT.

'use strict';

var _ = require('lodash')
  , util = require('util')
  , stream = require('stream')
  , Transform = stream.Transform
  , mmm = require('mmmagic')
  , Magic = mmm.Magic;

util.inherits(MimeChecker, Transform);

function MimeChecker(opts) {
  Transform.call(this, opts);
  this.data = [];
  this.magic = new Magic(mmm.MAGIC_MIME_TYPE);
}

MimeChecker.prototype._transform = function (chunk, encoding, done) {
  var self = this;

  if (self.mimeFound) {
    self.push(chunk);
    return done();
  }
 
  self.data.push(chunk);
  if (self.data.length < 20) {
    return done();
  }
  else if (self.data.length === 20) {
    self.pause();
    var buffered = Buffer.concat(this.data);
    this.magic.detect(buffered, function(err, result) {
      if (err) return self.emit('wrongmime', err);
      // we can also allow corrupt files that most of the time turn out to be ok
      if (!_.contains(self.getAllowedMimeTypes().concat(['application/CDFV2-corrupt']), result)) {
        return self.emit('wrongmime', null, result);
      }
      self.data.map(self.push.bind(self));
      self.mimeFound = true;
      self.emit('correctmime');
      return done();
    });
  }
};

MimeChecker.prototype.getAllowedMimeTypes = function () {
    // allow mp4 and webm videos only
    return [
      'video/mp4',
      'video/webm'
    ];
};

module.exports = MimeChecker;

usage.js

var file = getFileStreamSomehow();

var mimeChecker = new MimeChecker();

mimeChecker.on('wrongmime', function (err, mimetype) {
  file.unpipe();
  // abort the connection
});

mimeChecker.on('correctmime', function () {
  mimeChecker.resume();
  // pipe mimeChecker stream into your desired stream
  mimeChecker.pipe(outputStream);
});

// pipe file stream into the mimeChecker transform stream
file.pipe(mimeChecker);