I know there is a global setting for maximum size of a file that can be uploaded

There's no such setting for file types. Maybe because Appian platform has no restriction on supported file types.

AFAIK, the preferred way right now is to validate and loop back.

There's an alternate way, as someone in that discussion said, it's more of a hack than a solution: forum.appian.com/.../e-153370

I am also interested on setting the allowed file types