This option specifies the flash upload extensions which you don't wish to use in FCKeditor. Example:
FCKConfig.FlashUploadDeniedExtensions = ".(swf)$" ;
If you leave this option empty none extension will be denied.
NOTE: if you leave FlashUploadAllowedExtensions empty and you will put something in FlashUploadDeniedExtensions e.g.swf it will allow the upload of all the files exept the swf. However it isn't a good way to secure your server from unwanted uploads. The best way is to put all of the prefered extensions in FlashUploadAllowedExtensions and put the unwanted ones in the FlashUploadDeniedExtensions. Only in this way you will secure your server from junk.