This option specifies the link upload extensions which you don't wish to use in FCKeditor. Example:
FCKConfig.LinkUploadDeniedExtensions = ".(7z|rar|zip)$" ;
If you leave this option empty none extension will be denied.
NOTE: if you leave LinkUploadAllowedExtensions empty and you will put something in LinkUploadDeniedExtensions e.g.pdf it will allow the upload of all the files exept the pdf. 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 LinkUploadAllowedExtensions and put the unwanted ones in the LinkUploadDeniedExtensions. Only in this way you will secure your server from junk.