@Override
    protected String getSourceDisplayName() {
        return gpxFileName == null ? ".gpx" : TextUtils.stripHtml(gpxFileName);
    }
Get the file size of a specific file inside the zip archive.
It worth it to spend some extra time to gather the size so that we can visualize a progress while parsing afterwards.
This method is rather time consuming, so don't call this if the size can also be found otherwise.

@param filename the filename of the file inside the zip archive.
@return the uncompressed size of the entry data
main/src/main/java/cgeo/geocaching/files/AbstractImportGpxZipThread.java