I thought it might by simple (FCKConfig.LinkUploadAllowedExtensions = docx|xlsx in FCKeditor/custom_configurations/fckconfig_cmsimple.js) but no way, neighter in FCKeditor/fckconfig.js.
line 152: $Config['AllowedExtensions']['File'] =..
line 180: $Config['AllowedExtensions']['Downloads']=..
Great if the extensions (MS Office 2007 & up) will be actualized in the next release.
It's a bit confusing where to set the allowed extension.