I have a few nationalities in me: British, Dutch, Native American, Chinese to name a few.
I believe the term is "Afghani". Similar to "Irani" and "Pakistani". Stupid Western media created their own terms like "Iranian" (pronounce eye-rain-eeyan).
That word was in use long before the "media" was an entity.
The "stupid western media" did, however, coin the term "Islamic" to replace the proper English adjective "Mohammedan". Apparently, it's less insulting.
Never, ever, ever, ever heard that term in my life, and I know a fair bit about Islam's history. Never called ourselves "Mohammeden" or anything like that.
That said, I never refer to myself as "Islamic", I just call myself "Muslim", which is the correct term.
So, you've never read a book written before the 1960s?
What Muslims call themselves is not necessarily what is proper and correct in English.
After the Six Day War, and with the rise of militant Islam, the newspapers invented the adjective "Islamic". Because they're *******.