MIA currently only supports '.MP3' but I will probably be adding more music types in future. I'm slightly limited by the file types that DBPro lets me use.

Editing of the game is done through the editors, but to make models and such you will need other software that I do not have the skill to make.
Now before you whine that I am making you shell out hard cash I would like you to search online for:

Blender: A totaly free and very powerful modeling tool
Milkshape: A £20 and pretty simple modeling tool (nice though !)
GIMP: A totaly free piece of image editing software.
Audacity: A totaly free piece of sound/music editting software, brilliant piece of kit.

There are options. If you don't like that idea then I guess you'll just have to make a team with a modeler, wait for models to be released for free... or scoure the web for free models and textures.
Personal I recommend looking at
Turbosquid (google it !)
and there are so many free texture sites out there I have lost count.

So yeah thats how you edit the game !