It is whitelist, like any other one, when the new (Discourse) software was installed, the whitelist did not include some commonly used formats, see:
Migration process from our old phpBB instance to Discourse is now done - #51 by jaclaz
Originally it was only:
(authorized extensions: jpg, jpeg, png, gif, heic, heif, webp, avif)
krisjanis added, upon request, the other extensions.