To follow up on Yenchi's comment above, the OK button will also do nothing if the camera app can't write to the directory in question.
That means that you can't create the file in a place that's only writeable by your application (for instance, something under getCacheDir())
Something under getExternalFilesDir()
ought to work, however.
It would be nice if the camera app printed an error message to the logs if it could not write to the specified EXTRA_OUTPUT
path, but I didn't find one.