An _XLSX file is a Microsoft Excel spreadsheet file renamed from ".xlsx" to "._xlsx". It may have been renamed purposely or accidentally by a user or by a program. _XLSX files contain the same contents of an .XLSX file, which may include one or more worksheets that consist of cells arranged in a grid of rows and columns.
You can rename an _XLSX file by changing the "._xlsx" extension to ".xlsx". After renaming the file, you can open the XLSX file with a variety of programs, such as Microsoft Excel, LibreOffice Calc, and Apple Numbers.
_XLSX files may be created by email programs, specifically Windows Live Mail, when sending XLSX files via email. Windows Live Mail may rename the files if the filename ends with a "." before the "." in the extension. For example, sample..xlsx is changed to sample._xlsx by the email program. This also commonly occurs with .DOC, .DOCX, and .XLS files.
NOTE: XLSX files are typically created by Microsoft Excel. However, they may also be created using another spreadsheet program, such as LibreOffice Calc or Apple Numbers.