A file with an .ai extension is an Adobe Illustrator Artwork file that contains vector graphics on a single page. It uses points to create paths for displaying the image data, thus making it safe from losing image quality if it is enlarged. AI format finds its major usage for logos and print media. AI files can be opened with Adobe Illustrator, Adobe Acrobat DC, PaintShop Pro, and CorelDraw Graphics tools.
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 convert AI into many other file formats. Please see the complete list below.