@Nullable
    public DownloaderUtils.DownloadDescriptor getExtrafile(final String[] filenames, final String baseUrl, final Download.DownloadType offlineMapType) {
        final List<ContentStorage.FileInformation> dirContent = ContentStorage.get().list(PersistableFolder.OFFLINE_MAP_THEMES);
        for (ContentStorage.FileInformation fi : dirContent) {
            for (String filename : filenames) {
                if (fi.name.equals(filename)) {
                    return null;
                }
            }
        }
        return new DownloaderUtils.DownloadDescriptor(filenames[0], Uri.parse(baseUrl + filenames[0]), offlineMapType.id);
    }
Check if any of the given files already exists in the given path

@param filenames
     Array of filenames to check for
@param baseUrl
     Base URL for the first filename
@param offlineMapType
     Descriptor type to return
@return
     Returns null, if any of the given files found.
     Returns a download descriptor for the first entry (so baseUrl must be valid for that entry)
main/src/main/java/cgeo/geocaching/downloader/AbstractMapDownloader.java