[IMP] mail: Add a preview popup for attachments

Purpose
=======

The goal is that when you click on an attachment, you should get a preview of it and then you get the possibility to download it.

addons/web/static/src/less/mimetypes.less

@@ -2,7 +2,7 @@
     background-image: url('/web/static/src/img/mimetypes/unknown.png');
     background-size: contain;
     width: 100px;
-    height: 80px;
+    height: 100px;
     display: inline-block;
     background-position: center;
     background-repeat: no-repeat;