It used to be you just put what file extension you wanted in the save dialog and gimp would save the that file format. Honestly I likes it, I always thought it was one of saner ways to handle a complex subject.
However. the gimp native format is .xcf xcf is the only format that will save all of gimps internal data. due to the ease of loosing data it was decided to split the save system into two parts "save" only saves in xcf format, all other formats are exported.
so no, no longer can you just work on a png, gimp will nag you to save to xcf every time, I understand the change but I can't help but think that we lost something along the way. something about tools that stay out of your way and so what you want of them.
