Handle GIF attachments

Not sure whether providing a download link for attached images is the
right thing to do - we should probably just display them like inline
images. But we can always change that by a simple change of the
template.

templates/attachment_image_gif.html

@@ -0,0 +1,8 @@
+<div class="partouter">
+    <div class="partheader">
+        image/gif
+    </div>
+    <div class="partinner">
+        <a href="{{url}}">{{name}}</a>
+    </div>
+</div>