Actually, the most common expression would be: "mon ami(e)".
If there's nothing else that define that person (ex: "mon ami japonais"), people will assume that he's your partner ("mon" makes that person unique, and since, as I said above, no other information has been added, that person is unique to you because it's your boyfriend).
For that very reason, if you have some activities with someone who's just a friend, avoid to use "mon ami(e)". Use, "un ami / une ami" instead to avoid ambiguity.
"J'ai diné avec un(e) ami(e)"
:)
ps. I agree with the other answers