File Manipulation

Question for the masters…
Using AS2 or 3, is there a way to create my own file format?

I’m having a hard time using the correct words to explain what I want to do.

Let me try to give an example.
I would like to create a game.
And when users play the game, they can create a GAME-SAVE-FILE.
So, let’s say that player 1 is playing… He get’s to level 22 and has to sign off.
He clicks save my GAME-SAVE-FILE to my computer.
He names the file MyGame and saves it to his desktop.
So now on his desktop he has MyGame.gsf

Next time he visits, he can click on UpLoad My Saved Game.
He selects his previously saved MyGame.gsf
And bammo… all the information was stored in his file and now he can play on.

I need to know how I would put together the format for my own file-type / extension.
I need to know how I would access, write, and edit my own file-type / extension.
And I can’t find the information for this anywhere.

I am fairly certain it is possible.
Since Flash can do this type of File Manipulation with PNG’s and JPG’s.
So surely it can be done with any desired custom format.
Anyone know where to point me???