Improve the file delegate

- cleaner code
- supports stopping downloads
- utilises the mimetype in order to display an icon
- better size formatting
- tooltips

src/messageeventmodel.h

@@ -30,6 +30,8 @@ public:
         UserMarkerRole,
         FormattedBodyRole,
 
+        FileMimetypeIcon,
+
         ReplyRole,
 
         ShowAuthorRole,