Add a custom taxonomy dropdown filter to the WordPress Media Library

collection-filter.js

(function(){
	/**
	 * Create a new MediaLibraryTaxonomyFilter we later will instantiate
	 */
	var MediaLibraryTaxonomyFilter = wp.media.view.AttachmentFilters.extend({
		id: 'media-attachment-taxonomy-filter',

		createFilters: function() {
			var filters = {};
			// Formats the 'terms' we've included via wp_localize_script()
			_.each( MediaLibraryTaxonomyFilterData.terms || {}, function( value, index ) {
				filters[ index ] = {
					text: value.name,
					props: {
						// Change this: key needs to be the WP_Query var for the taxonomy
						collection: value.slug,
					}
				};
			});
			filters.all = {
				// Change this: use whatever default label you'd like
				text:  'All collections',
				props: {
					// Change this: key needs to be the WP_Query var for the taxonomy
					collection: ''
				},
				priority: 10
			};
			this.filters = filters;
		}
	});
	/**
	 * Extend and override wp.media.view.AttachmentsBrowser to include our new filter
	 */
	var AttachmentsBrowser = wp.media.view.AttachmentsBrowser;
	wp.media.view.AttachmentsBrowser = wp.media.view.AttachmentsBrowser.extend({
		createToolbar: function() {
			// Make sure to load the original toolbar
			AttachmentsBrowser.prototype.createToolbar.call( this );
			this.toolbar.set( 'MediaLibraryTaxonomyFilter', new MediaLibraryTaxonomyFilter({
				controller: this.controller,
				model:      this.collection.props,
				priority: -75
			}).render() );
		}
	});
})()

enqueue.php

<?php

add_action( 'wp_enqueue_media', function() {
	wp_enqueue_script( 'media-library-taxonomy-filter', get_stylesheet_directory_uri() . '/assets/js/collection-filter.js', array( 'media-editor', 'media-views' ) );
	// Load 'terms' into a JavaScript variable that collection-filter.js has access to
	wp_localize_script( 'media-library-taxonomy-filter', 'MediaLibraryTaxonomyFilterData', array(
		'terms'     => get_terms( 'collection', array( 'hide_empty' => false ) ),
	) );
	// Overrides code styling to accommodate for a third dropdown filter
	add_action( 'admin_footer', function(){
		?>
		<style>
		.media-modal-content .media-frame select.attachment-filters {
			max-width: -webkit-calc(33% - 12px);
			max-width: calc(33% - 12px);
		}
		</style>
		<?php
	});
});