Don't trigger HTML_SHORT_LINK_IMG on small images

It's common for legitimate emails to have small image links (buttons, social networks junk e.t.c.)

rules/html.lua

@@ -38,7 +38,10 @@ local function check_html_image(task, min, max)
               local parent = tag:get_parent()
               if parent then
                 if parent:get_type() == 'a' then
-                  return true
+                  -- do not trigger on small and unknown size images
+                  if i['height'] + i['width'] >= 210 then
+                    return true
+                  end
                 end
               end
             end