How to create thumbnails of videos with Meteor, CollectionFS, & FFMPEG

Question

I'm using Meteor with CollectionFS to store videos. I need a transform to create thumbnails of my videos.

Videos = new FS.Collection("videos", {
  stores: [
    new FS.Store.FileSystem("thumbs", { 
      transformWrite: function(fileObj, readStream, writeStream) {
        // What goes here?
      }
    }),
    new FS.Store.FileSystem("videos"),
  ],
});

I've worked out how to do this with ffmpeg:

ffmpeg -i video.mp4 -vf  "thumbnail,scale=640:360" -frames:v 1 thumb.png

But I'm not sure how to do this with the readStream I'm given and output the writeStream.

Here's an example of how it is done with images using GraphicsMagick:

Images = new FS.Collection("images", {
  stores: [
    new FS.Store.FileSystem("thumbs", { 
      transformWrite: function(fileObj, readStream, writeStream) {
        // Transform the image into a 10x10px thumbnail
        gm(readStream, fileObj.name()).resize('10', '10').stream().pipe(writeStream);
      }
    }),
    new FS.Store.FileSystem("images"),
  ],
});

Although the samples use the local filesystem, Ill be using cvs:dropbox, so you can't rely on the file being there locally.