From FCKeditor Docs

FlashUploadURL

This option defines the URL to post files through the "Upload Tab" in the flash dialog. It can point to the default file uploader:

FCKConfig.FlashUploadURL = FCKConfig.BasePath + 'filemanager/connectors/' + _QuickUploadLanguage + '/upload.' + _QuickUploadExtension ;

or any custom implementation of it:

FCKConfig.FlashUploadURL = '/myuploader/upload.php';