An _XLS file is a Microsoft Excel spreadsheet file renamed from ".xls" to "._xls". It may have been renamed by a user on purpose or by accident or another program. _XLS files contain the contents of an .XLS file, which may include one or more worksheets, which store and display data in a table format.
You can rename an _XLS file by changing the "._xls" extension to ".xls". After renaming the file, you can open the XLS file with a variety of programs, such as Microsoft Excel and LibreOffice Calc.
_XLS files are sometimes created by email programs, specifically Windows Live Mail, when sending an XLS file via email. Windows Live Mail may rename it if the filename ends with a "." before the "." in the extension. For example, sample..xls is changed to sample._xls by the email program. This also commonly occurs with .DOC, .DOCX, and .XLSX files.
NOTE: XLS files are typically created by Microsoft Excel. However, they may also be created using another spreadsheet program, such as LibreOffice Calc or Apple Numbers.