Mage_Cms_Helper_Wysiwyg_Images has a bug when you have the /media directory symlinked in to the deployment location (At least as of EE 1.13.1)

Fixed.php

<?php
/**
 * Return URL based on current selected directory or root directory for startup
 *
 * @return string
 */
public function getCurrentUrl()
{
    if (!$this->_currentUrl) {
        $mediaDir = Mage::getConfig()->getOptions()->getMediaDir();
        $path = str_replace(realpath($mediaDir), '', $this->getCurrentPath());
        $path = trim($path, DS);
        $this->_currentUrl = Mage::app()->getStore($this->_storeId)->getBaseUrl('media') . $this->convertPathToUrl($path) . '/';
    }
    return $this->_currentUrl;
}

Original.php

<?php
/**
 * Return URL based on current selected directory or root directory for startup
 *
 * @return string
 */
public function getCurrentUrl()
{
    if (!$this->_currentUrl) {
        $path = str_replace(Mage::getConfig()->getOptions()->getMediaDir(), '', $this->getCurrentPath());
        $path = trim($path, DS);
        $this->_currentUrl = Mage::app()->getStore($this->_storeId)->getBaseUrl('media') .
                             $this->convertPathToUrl($path) . '/';
    }
    return $this->_currentUrl;
}