remove cut/paste error which results in a server error. is not defined when there are no assets. Also fix up base.js to only register one set of handlers for file section which gets rid of the multiple file selection dialogs from appearing

cms/templates/asset_index.html

@@ -99,7 +99,7 @@
 			</div>
       <div class="embeddable">
         <label>Embeddable XML:</label>
-        <input type="text" class="embeddable-xml-input" value='<img src="${asset['url']}"/>'>
+        <input type="text" class="embeddable-xml-input" value='<img src=""/>'>
       </div>
 			<form class="file-chooser" action="${upload_asset_callback_url}"
                                 method="post" enctype="multipart/form-data">