Some people would say "gloss", but that is because everyone at car shows slaps Armor All on the metal parts of the door interior.
I would say that it's a satin texture, leaning towards a gloss.
The texture of the door affects the look of the paint. The interior paint in cans for Mustangs works best. It's definately not flat.
if you can, look at Mustang brochures, or photos of Mustang interiors from old print ads. You can tell from those original photos.
Paul.