Files with .XLT extension are template files created with Microsoft Excel which is a spreadsheet application which comes as part of Microsoft Office suite. Microsoft Office 97-2003 supported creating new XLT files as well as opening these. The latest version of Excel is still capable of opening these old format template files. Such a template file is used to quickly create new Excel files with default data and settings such as page formatting, font size, margins, charts, etc which can be further saved as new .XLS files.
You can also add watermark to many other file formats. Please see the complete list below.