void addExtension(String extension) {
        if (extensions == null) {
            extensions = Collections.singletonList(extension);
        } else if (extensions.size() == 1) {
            extensions = new ArrayList<>(extensions);
        }
        if (!extensions.contains(extension)) {
            extensions.add(extension);
        }
    }
Adds a known file extension to this type.

@param extension file extension
tika-core/src/main/java/org/apache/tika/mime/MimeType.java