Index: src/wp-includes/css/media-views.css
===================================================================
--- src/wp-includes/css/media-views.css	(revision 29038)
+++ src/wp-includes/css/media-views.css	(working copy)
@@ -2686,6 +2686,7 @@
 .edit-attachment-frame .attachment-info .thumbnail {
 	max-width: none;
 	max-height: none;
+	width: 100%;
 }
 
 .edit-attachment-frame .attachment-info .thumbnail-image img {
@@ -2698,7 +2699,7 @@
 }
 
 .edit-attachment-frame .attachment-info .thumbnail img {
-	max-width: none;
+	max-width: 100%;
 	max-height: 50%;
 }