[Minor] Lua_magic: Improve html detection heuristic

lualib/lua_magic/heuristics.lua

@@ -435,10 +435,17 @@ exports.text_part_heuristic = function(part, log_obj, _)
         end
       end
 
+      -- Content type stuff
       if (mtype == 'text' or mtype == 'application') and (msubtype == 'html' or msubtype == 'xhtml+xml') then
         return 'html',21
       end
 
+      -- Extension stuff
+      local fname = part:get_filename()
+      if fname and fname:match('html?$') then
+        return 'html',21
+      end
+
       return 'txt',40
     end
   end