Query all attachments uploaded to a post.

gallery.js

// Build a query for all attachments uploaded to a post.
var gallery = wp.media.query({ uploadedTo: postId });

// Run the query.
// This returns a promise (like $.ajax) so you can do things when it completes.
gallery.more();

// Bind your events for when the contents of the gallery changes.
gallery.on( 'add remove reset', function() {
	// Something changed, update your stuff.
}, gallery );