Index: src/wp-includes/js/media-editor.js
===================================================================
--- src/wp-includes/js/media-editor.js	(revision 37557)
+++ src/wp-includes/js/media-editor.js	(working copy)
@@ -55,7 +55,13 @@
 			fallbacks = function( props ) {
 				// Generate alt fallbacks and strip tags.
 				if ( 'image' === props.type && ! props.alt ) {
-					props.alt = props.caption || props.title || '';
+					if( props.caption ) {
+						props.alt = props.caption;
+					} else if( props.title !== props.url ){
+						props.alt = props.title;
+					} else {
+						props.alt = '';	
+					}
 					props.alt = props.alt.replace( /<\/?[^>]+>/g, '' );
 					props.alt = props.alt.replace( /[\r\n]+/g, ' ' );
 				}
@@ -232,7 +238,8 @@
 		image: function( props, attachment ) {
 			var img = {},
 				options, classes, shortcode, html;
-
+			
+			props.type = 'image';
 			props = wp.media.string.props( props, attachment );
 			classes = props.classes || [];