.xlsx, .docx, .pptx contains extensions of MS-Office files.
XLSX – is an Excel workbook.
DOCX is part of Microsoft Office Open XML specification (also known as OOXML or Open XML) and was introduced with Office 2007. DOCX is azipped, XML-based file format. Microsoft Word 2007 and later use DOCX as the default file format when creating a new document. Support for loading and saving legacy DOC files is also included (Docx – Word document).
PPTX – is a PowerPoint Open XML Presentation file.
Office Open XML (OOXML) format was introduced with Microsoft Office 2007 and became the default format of Microsoft Excel ever since. Excel-related file extensions of this format include: .xlsx, .xlsm, .xltx, .xltm.