Re-reading my post I see that I have sent an incorrect example. Sorry!
So the right example here:
The URLs of my subdomain's attachments seems like this:
http://exampleMAINdomain.com/wp-content/uploads/sites/3/2016/02/sample.jpg
I would like change it to:
http://exampleSUBdomain.com/wp-content/uploads/2016/02/sample.jpg
Do you recommend any method for this URL transform?
Kind regards,
Jozsef