public static ArrayList<DownloadedFileData> availableOfflineMaps(@NonNull final Download.DownloadType filter) {
        final ArrayList<DownloadedFileData> result = new ArrayList<>();
        final AbstractDownloader downloader = Download.DownloadType.getInstance(filter.id);

        final List<ContentStorage.FileInformation> files = ContentStorage.get().list(downloader.targetFolder);
        final Map<String, Uri> filesMap = new HashMap<>();
        for (ContentStorage.FileInformation fi : files) {
            filesMap.put(fi.name, fi.uri);
        }

        if (downloader.useCompanionFiles) {
            for (ContentStorage.FileInformation fi : files) {
                final String filename = fi.name;
                final DownloadedFileData offlineMapData = readData(downloader.targetFolder.getFolder(), filename);
                if (offlineMapData != null && offlineMapData.remoteParsetype == filter.id && StringUtils.isNotBlank(offlineMapData.localFile) && filesMap.containsKey(offlineMapData.localFile)) {
                    result.add(offlineMapData);
                }
            }
        } else {
            for (ContentStorage.FileInformation fi : files) {
                if (!fi.name.endsWith(INFOFILE_SUFFIX) && fi.name.endsWith(downloader.forceExtension)) {
                    final DownloadedFileData download = new DownloadedFileData();
                    download.remoteParsetype = filter.id;
                    download.remoteDate = fi.lastModified;
                    download.remoteFile = fi.name;
                    download.localFile = fi.name;
                    download.displayName = fi.name;
                    // some properties remain unset when not using companion files (remotePage), others are guesses only (remoteFile, remoteParseType)
                    result.add(download);
                }
            }
        }
        return result;
    }
returns a list of downloaded offline files from requested source which are still available in the local filesystem
main/src/main/java/cgeo/geocaching/downloader/CompanionFileUtils.java