Index: wp-includes/js/media-editor.js
===================================================================
--- wp-includes/js/media-editor.js	(revision 23084)
+++ wp-includes/js/media-editor.js	(working copy)
@@ -555,7 +555,8 @@
 
 				options = {
 					id:           attachment.id,
-					post_content: attachment.description
+					post_content: attachment.description,
+					post_excerpt: caption,
 				};
 
 				if ( props.linkUrl )
@@ -563,7 +564,6 @@
 
 				if ( 'image' === attachment.type ) {
 					html = wp.media.string.image( props );
-					options.post_excerpt = caption;
 
 					_.each({
 						align: 'align',
Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 23084)
+++ wp-includes/media.php	(working copy)
@@ -1751,6 +1751,10 @@
 				<# } #>
 				/>
 			</label>
+			<label class="setting" data-setting="caption">
+				<span><?php _e('Caption'); ?></span>
+				<textarea {{ maybeReadOnly }}>{{ data.caption }}</textarea>
+			</label>
 		<# } #>
 			<label class="setting" data-setting="description">
 				<span><?php _e('Description'); ?></span>