public boolean addExtension(String extension) {
        if (extension.startsWith("."))
            throw new IllegalArgumentException("Invalid file extension format. You must " +
                                               "start the extension with a period (.), " +
                                               "followed by the actual extension itself. " +
                                               "Exception thrown for the following extension: " +
                                                extension);
        return getFilterMap().put(extension, false);
    }
Adds the specified extension to the HashMap. The extension MUST be
in the following format: ".xxx", where xxx is the actual file extension.
If the format doesn't match up, the extension will not be added to the
HashMap.

@param extension The extension String to add to the HashMap.
@return True, if the extension was successfully added to the HashMap. False otherwise.
core/src/main/java/com/filebrowser/FileBrowserEngine/FileExtensionFilter.java