What type of aluminium to use for decorative trim?
1/2" by 1/2" by X (long) aluminum angle with an eased outside edge if you can find it, and countersunk by you or held in with adhesive.



If it's going to look exactly like that picture, you cannot use MDF there.


  ... combustible materials shall not be in contact with the masonry or concrete walls less than 12 inches (305 mm)from the inside surface of the nearest firebox lining. –What is a fireplace facing and what is it used for? 


E.g., (even in this picture they're cutting corners)



"The decorative trim will not be particularly close to the fire." - don't care. The trim needs to be a certain distance away from the opening.

Keep combustibles (drywall, MDF, etc.) one foot from the opening in every direction. For applicable exceptions, see the linked question.



See also, How much clearance is needed on a gas fireplace?