Common file extensions. It can be helpful to know some of the most common file extensions that you might use. Here is a list of some of the file extensions and what they mean. Text Files: .txt – A text file, .exe – An executable file, . log – A log file, .doc – A Microsoft Word document, .docx – A Microsoft Word Open XML document, .msg – An Outlook Mail Message, .pages – A Pages document, .rtf – A Rich Text File.