diff --git a/src/js/media/models/attachments.js b/src/js/media/models/attachments.js
index 1683494388283..fb31ba09ab9d6 100644
--- a/src/js/media/models/attachments.js
+++ b/src/js/media/models/attachments.js
@@ -32,6 +32,8 @@ var Attachments = Backbone.Collection.extend(/** @lends wp.media.model.Attachmen
 	 * @param {Object} [options={}]
 	 */
 	initialize: function( models, options ) {
+		var normalizedOrder;
+
 		options = options || {};
 
 		this.props   = new Backbone.Model();
@@ -44,7 +46,19 @@ var Attachments = Backbone.Collection.extend(/** @lends wp.media.model.Attachmen
 		this.props.on( 'change:orderby', this._changeOrderby, this );
 		this.props.on( 'change:query',   this._changeQuery,   this );
 
-		this.props.set( _.defaults( options.props || {} ) );
+		options.props = options.props || {};
+
+		/*
+		 * Normalize the order, if one is set. `Attachments.comparator()` and the
+		 * `order` filter in `wp.media.model.Query` both test for the literal
+		 * strings 'ASC' and 'DESC', so anything else has to fall back to 'DESC'.
+		 */
+		if ( ! _.isUndefined( options.props.order ) && ! _.isNull( options.props.order ) ) {
+			normalizedOrder     = String( options.props.order ).toUpperCase();
+			options.props.order = ( 'ASC' === normalizedOrder || 'DESC' === normalizedOrder ) ? normalizedOrder : 'DESC';
+		}
+
+		this.props.set( options.props );
 
 		if ( options.observe ) {
 			this.observe( options.observe );
diff --git a/src/js/media/models/query.js b/src/js/media/models/query.js
index b3f62018f5cd4..3c47215c39833 100644
--- a/src/js/media/models/query.js
+++ b/src/js/media/models/query.js
@@ -251,12 +251,6 @@ Query = Attachments.extend(/** @lends wp.media.model.Query.prototype */{
 			// Fill default args.
 			_.defaults( props, defaults );
 
-			// Normalize the order.
-			props.order = props.order.toUpperCase();
-			if ( 'DESC' !== props.order && 'ASC' !== props.order ) {
-				props.order = defaults.order.toUpperCase();
-			}
-
 			// Ensure we have a valid orderby value.
 			if ( ! _.contains( orderby.allowed, props.orderby ) ) {
 				props.orderby = defaults.orderby;
diff --git a/tests/qunit/index.html b/tests/qunit/index.html
index a6b6177014586..d0e81acedb502 100644
--- a/tests/qunit/index.html
+++ b/tests/qunit/index.html
@@ -152,6 +152,7 @@
 		<script src="wp-admin/js/customize-header.js"></script>
 		<script src="wp-admin/js/dashboard.js"></script>
 		<script src="wp-includes/js/shortcode.js"></script>
+		<script src="wp-includes/js/media/test-media-models.js"></script>
 		<script src="wp-includes/js/api-request.js"></script>
 		<script src="wp-includes/js/jquery.js"></script>
 		<script src="wp-includes/js/wp-api.js"></script>
diff --git a/tests/qunit/wp-includes/js/media/test-media-models.js b/tests/qunit/wp-includes/js/media/test-media-models.js
new file mode 100644
index 0000000000000..b6d5563f584b6
--- /dev/null
+++ b/tests/qunit/wp-includes/js/media/test-media-models.js
@@ -0,0 +1,254 @@
+/* globals wp */
+/* jshint qunit: true */
+/* eslint-env qunit */
+/* eslint-disable no-magic-numbers */
+
+( function() {
+	'use strict';
+
+	QUnit.module( 'Media Models - Order Normalization' );
+
+	// Test valid uppercase values
+	QUnit.test( 'Attachments should accept uppercase "ASC" order', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 'ASC'
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'ASC',
+			'Order should remain ASC when passed as uppercase' );
+	});
+
+	QUnit.test( 'Attachments should accept uppercase "DESC" order', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 'DESC'
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Order should remain DESC when passed as uppercase' );
+	});
+
+	// Test lowercase normalization
+	QUnit.test( 'Attachments should normalize lowercase "asc" to uppercase', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 'asc'
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'ASC',
+			'Order should be converted from lowercase asc to uppercase ASC' );
+	});
+
+	QUnit.test( 'Attachments should normalize lowercase "desc" to uppercase', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 'desc'
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Order should be converted from lowercase desc to uppercase DESC' );
+	});
+
+	// Test mixed case normalization
+	QUnit.test( 'Attachments should normalize mixed case "AsC" to uppercase', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 'AsC'
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'ASC',
+			'Order should be converted from mixed case AsC to uppercase ASC' );
+	});
+
+	QUnit.test( 'Attachments should normalize mixed case "DeSc" to uppercase', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 'DeSc'
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Order should be converted from mixed case DeSc to uppercase DESC' );
+	});
+
+	// Test invalid string values
+	QUnit.test( 'Attachments should default invalid string order to "DESC"', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 'invalid'
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Invalid string order should default to DESC' );
+	});
+
+	QUnit.test( 'Attachments should default empty string order to "DESC"', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: ''
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Empty string order should default to DESC' );
+	});
+
+	/*
+	 * An unset order is left alone so the existing 'DESC' fallbacks in
+	 * Attachments.comparator() still apply. Any value that *is* set gets
+	 * normalized, otherwise a truthy non-string would sort ascending.
+	 */
+	QUnit.test( 'Attachments should leave a null order value unset', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: null
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), null,
+			'Null order should remain null' );
+	});
+
+	QUnit.test( 'Attachments should leave an undefined order value unset', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: undefined
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), undefined,
+			'Undefined order should remain undefined' );
+	});
+
+	QUnit.test( 'Attachments should default a numeric order to "DESC"', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 123
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Numeric order should default to DESC' );
+	});
+
+	QUnit.test( 'Attachments should default a boolean true order to "DESC"', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: true
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Boolean true order should default to DESC' );
+	});
+
+	QUnit.test( 'Attachments should default a boolean false order to "DESC"', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: false
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Boolean false order should default to DESC' );
+	});
+
+	QUnit.test( 'Attachments should default an object order to "DESC"', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: { value: 'ASC' }
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Object order should default to DESC' );
+	});
+
+	QUnit.test( 'Attachments should default an array order to "DESC"', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: ['ASC', 'DESC']
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Array order should default to DESC' );
+	});
+
+	// Test when no order property is provided
+	QUnit.test( 'Attachments should work when no order property is provided', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				orderby: 'date'
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), undefined,
+			'Order should be undefined when not provided' );
+	});
+
+	/*
+	 * Query no longer normalizes the order itself, it relies on inheriting the
+	 * normalization above. Note these pass `args` rather than `props.query`:
+	 * setting `query` would kick off a server request via `_requery()`.
+	 */
+	QUnit.test( 'Query should inherit order normalization from Attachments', function( assert ) {
+		var query = new wp.media.model.Query( [], {
+			props: {
+				order: 'asc'
+			},
+			args: {}
+		});
+
+		assert.strictEqual( query.props.get('order'), 'ASC',
+			'Query model should normalize order through inheritance from Attachments' );
+		assert.ok( query instanceof wp.media.model.Attachments,
+			'Query should be instance of Attachments' );
+	});
+
+	QUnit.test( 'Query should default invalid order to "DESC"', function( assert ) {
+		var query = new wp.media.model.Query( [], {
+			props: {
+				order: 'random'
+			},
+			args: {}
+		});
+
+		assert.strictEqual( query.props.get('order'), 'DESC',
+			'Query model should default invalid order to DESC' );
+	});
+
+	// Test whitespace handling
+	QUnit.test( 'Attachments should handle order with whitespace', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: '  asc  '
+			}
+		});
+
+		assert.notStrictEqual( collection.props.get('order'), 'ASC',
+			'Order with whitespace should not match ASC exactly' );
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Order with whitespace should default to DESC as it does not match ASC/DESC after toUpperCase' );
+	});
+
+	// Test unicode characters
+	QUnit.test( 'Attachments should handle order with unicode characters', function( assert ) {
+		var collection = new wp.media.model.Attachments( [], {
+			props: {
+				order: 'asc\u200B'  // Zero-width space
+			}
+		});
+
+		assert.strictEqual( collection.props.get('order'), 'DESC',
+			'Order with unicode characters should default to DESC' );
+	});
+
+})();