public void selectMultipleFiles(@Nullable final String type, @Nullable final Uri startUri) {
        selectFilesInternal(type, startUri, SelectAction.SELECT_FILE_MULTIPLE, null);
    }
Asks user to select multiple files at once
if a callback for action {@link SelectAction#SELECT_FILE_MULTIPLE} is registered, it will be called after selection has finished
main/src/main/java/cgeo/geocaching/storage/ContentStorageActivityHelper.java