A file with ._XLS extension is actually an XLS file that has been renamed to _XLS by some other applications. It is based on XLS file format structure, and if renamed to .xls, can be opened with all the applications supporting the XLS file format. In most cases, _XLS files are generated with XLS file attached to an email that converts it _XLS at the receiving end. Inside these, the use the same XLS File Formation Specifications to contain different type of information such as charts, graphs, and spreadsheet data.
_XLS File Format - More information
_XLS files are stored to disc in binary file format and can be opened in Microsoft Word by changing its extension to XLS. The complete file format specifications of _XLS file format are available publicly by Microsoft for developer’s reference.