Method _getFilename

/**
 * Determine the appropriate 'filename' of a file
 *
 * @param   string   $filename   Original filename of the file
 * @param   string   $mode       Mode string to retrieve the filename
 * @param   boolean  $usePrefix  Controls the use of a prefix
 * @param   boolean  $relative   Determines if the filename given is relative. Relative paths do not have JPATH_ROOT stripped.
 *
 * @return  string
 *
 * @since   1.7.0
 */
public function _getFilename($filename, $mode, $usePrefix, $relative)
{
    if ($usePrefix) {
        // Get rid of binary or t, should be at the end of the string
        $tmode = trim($mode, 'btf123456789');
        // Check if it's a write mode then add the appropriate prefix
        // Get rid of JPATH_ROOT (legacy compat) along the way
        if (\in_array($tmode, FilesystemHelper::getWriteModes())) {
            if (!$relative && $this->writeprefix) {
                $filename = str_replace(JPATH_ROOT, '', $filename);
            }
            $filename = $this->writeprefix . $filename;
        } else {
            if (!$relative && $this->readprefix) {
                $filename = str_replace(JPATH_ROOT, '', $filename);
            }
            $filename = $this->readprefix . $filename;
        }
    }
    return $filename;
}