fix overwrite req.body, add mimetype limit
fix overwrite req.body, add mimetype limit

now can mimetype limit
  bb.extend(app, {
            upload: true,
            path: './public/files',
            allowedPath: /./,
            limits: {
                fileSize: 10 * 1024 * 1024
            },
            mimeTypeLimit: ['image/jpeg', 'image/png']
    });
yahoo/express-busboy