Maybe you should find out first if it will be ok when the underwriter sees the extension was just filed after the stip was asked for.

In my experience with the type of merchants we deal with, most of the time they didn't "forget" to file the extension, I wouldn't be surprised if he wasn't even planning on filing period then when they're asked for it they say "I forgot to file". There are two things in life you just don't forget. Death and taxes.