The Filenames.getExtension(String) method would be better if it returned Optional rather than a nullable string. Create a Filenames.findExtension(String) method and deprecate the one that returns a nullable; update as many references as possible to the deprecated version.
If there are any similar or related methods (e.g. finding a filename extension of a URI), do the same for them.
Work for this ticket also deprecates a lot of content type lookups based upon filename extension, to be replaced in the future with some more general mechanism defined in an appropriate place.