I think that the diamond files will be slower. Try one before investing in a bunch.
If they were ‘a thing’, we would probably would have heard more about this application. I only hear about guys using them to touch up carbide tips, or, as Dremel bits.
Chainsaw files are designed specifically for saw chains: hardness, tooth pattern, etc. Some companies (Pferd, others?) list smooth and coarse versions in their catalog, but these may be hard to find.
I prefer the vitrified (‘pink’, etc.) grinding wheels, but have been cleaning my chains for a long time. Have taken a lot of **** in these forums for it.
I use a commercial, water-based cleaner containing sodium hydroxide (lye), such as SuperClean, some ZEP products, etc.
I had a long, pleasant conversation with a chemist / applications specialist at the other end of an ‘800’ number, listed on the back of one of those jugs. He convinced me that there was a benefit to the surfactants, and other stuff in the cleaners, over pure lye.
$10 / gallon at Menards, etc. Dilute 50:50 with water. Usually takes just a few minutes.
I will link some related threads.
Philbert
https://www.arboristsite.com/threads/philbert-meets-the-stihl-rs3.202969/
https://www.arboristsite.com/threads/philberts-chain-salvage-challenge.245369/
https://www.arboristsite.com/threads/chain-cleaning.72970/