WordPress/wp-includes/js/media/models/post-media.js

/**
 * Shared model class for audio and video. Updates the model after
 *   "Add Audio|Video Source" and "Replace Audio|Video" states return
 *
 * @constructor
 * @augments Backbone.Model
 */
var PostMedia = Backbone.Model.extend({
	initialize: function() {
		this.attachment = false;
	},

	setSource: function( attachment ) {
		this.attachment = attachment;
		this.extension = attachment.get( 'filename' ).split('.').pop();

		if ( this.get( 'src' ) && this.extension === this.get( 'src' ).split('.').pop() ) {
			this.unset( 'src' );
		}

		if ( _.contains( wp.media.view.settings.embedExts, this.extension ) ) {
			this.set( this.extension, this.attachment.get( 'url' ) );
		} else {
			this.unset( this.extension );
		}
	},

	changeAttachment: function( attachment ) {
		this.setSource( attachment );

		this.unset( 'src' );
		_.each( _.without( wp.media.view.settings.embedExts, this.extension ), function( ext ) {
			this.unset( ext );
		}, this );
	}
});

module.exports = PostMedia;