a dot followed by three letters, such as .doc or .jpg, that forms the end of the name of a computer document(计算机)文件扩展名(. 后跟三个字母,如 .doc 或 .jpg,形成的计算机文件扩展名)
Be sure to type in the document name and file extension before you click "save".在点“保存”前要输入文件名和扩展名。