For example, the docx extension tells your computer that Microsoft Word can open the file and to display a Word icon when you view it in File Explorer. In expenses.xlsx, for example, the first part of the file name is expenses and the extension is xlsx.Įxtensions tell your computer which application created or can open the file and which icon to use for the file. Windows file names have two parts separated by a period: first, the file name, and second, a three- or four-character extension that defines the file type.