_XLS is a file extension associated with files created by the popular Microsoft Excel application. _XLS files are identical to Excel’s default file format XLS. The added ‘_’is a part of practice to prevent malicious macros sometimes included in spreadsheets sent via email from damaging user’s computer.

In order to open _XLS files user must change its filename suffix to XLS. _XLS is nothing more than a standard spreadsheet document that stores data as cells organized in rows and columns.