Improve unsupported file type handling

Make arbitrary files look nicer and display the filename.

If an audio or video element fails to load for any reason, timeout after
a few seconds and render it as an arbitrary file. Also short circuit to
this treatment for common audio and video file types that we know are
going to fail, e.g., proprietary formats from apple.

// FREEBIE

background.html

@@ -151,6 +151,10 @@
         <img src='{{ source }}' class='preview' />
         <a class='x close' alt='remove attachment' href='#'></a>
   </script>
+  <script type='text/x-tmpl-mustache' id='file-view'>
+    <span class='icon'></span>
+    <span class='fileName' alt='{{ fileName }}' title='{{ altText }}'>{{ fileName }}</a>
+  </script>
   <script type='text/x-tmpl-mustache' id='hasRetry'>
     {{ messageNotSent }}
     <span href='#' class='retry'>{{ resend }}</span>