If you did not file the FBAR by April 18, you get an automatic extension to October 15. You don't have to do anything to get the extension. You have until October 15 to file your FBAR.
The FBAR is filed separately from your income tax return. It is not part of your tax return, so you don't amend your tax return to file the FBAR. I'm not sure what you mean by filing "with FBAR details." Do you mean that you answered the questions about foreign accounts on Schedule B? There is nothing in those questions about when you file the FBAR. If you answered the questions correctly there is no reason to amend your tax return.
Go to the following link on the IRS web site for more information about FBAR filing.
Report of Foreign Bank and Financial Accounts (FBAR)
