How do I set Mendely as the default citation manager?

Hello I was previously using Endnote and have changed over to Mendeley. I have just gone into a database and chosen a journal article to export into Mendeley and my first question is; What file type do I choose? and secondly if its the same as Endnote, how do I choose Mendeley as the default manager so Endnote doesn't automatically open when I hit export. Cheers J