Using Stream to return filename extensions in ResourceBundleLoader is probably overkill. (See CSAR-3.) It's probably best return a Set.
The name getFilenameExtensionSuffixes() may be unnecessarily confusing as well. We can consider "extension" to indicate the thing after the dot, and consider the dot to be the delimiter, not the extension itself.
Ah ha, I probably used the term "extension suffix" originally because ResourceBundle talks about "suffices", e.g. ResourceBundle.Control.toResourceName(String bundleName, String suffix). I think it's better to standardize on a single term "extension" in the GlobalMentor code, e.g. in the core com.globalmentor.io.Filenames, that is nevertheless allowed to contain a dot (e.g. an extension of foo.bar).