i used:
[[++original_site_url]]img/photo_leaves.jpg
on browser and on mobile version with "/en/" behind the url, it works good and i get:
http://siteurl.com/subfolder/img/photo_leaves.jpg
On the mobile version without "/en/" behind the url i get:
it loads the css though, it seems it just doesn't loads the mml template variables.
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i also tried using "site_url" instead of "original_site_url", then i get this on the browser/pc version:
http://siteurl.com/subfolder/en/img/photo_leaves.jpg
the strange thing is, i get the same url, but without "/en/" on the mobile version (while using the url without /en/):
http://siteurl.com/subfolder/img/photo_leaves.jpg