Syntax for Defender 'Ignore file types'?

Defender is locking out legitimate users because of the 'Favicon by Real Favicon Generator' plugin which adds a suffix to generated favicon URLs to prevent browsers viewing cached versions.

However, users are being locked out of the site for 404 errors of favicon requested with old (presumably cached) suffixes which no longer exist.

I see there's a 'Ignore file types' option in Defender, to which I'd like to add PNG and ICO file extensions. But there's nothing with the option explaining the syntax or an example.

Is this comma separated? Semi-colon separated? Spaces? One per line?

Information would be handy, but updating the plugin with some guidance would be better UX!