Sometimes when you are uploading a file it may contain HTML code in the first kilobytes of its data. CKFinder will upload the file with the HTML code only when the file extension is added to the following configuration setting:
HtmlExtensions = new string[] { "html", "htm", "xml", "js" };
If you, for example, want to upload an .xsl file that contains HTML code at the beginning of the file, you should add this file extension to the list.
HtmlExtensions = new string[] { "html", "htm", "xml", "js", "xsl" };
Please note that this feature performs only a very basic set of checks to detect HTML-like data in the first 1kB of the file contents to protect users e.g. against unintentional uploading of files with HTML content and with a wrong extension.