file extension
noun [ C ] uk/ˈfaɪl ɪkˌsten.ʃən/ us/ˈfaɪl ɪkˌsten.ʃən/
（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）
a dot followed by three letters, such as .doc or .jpg, that forms the end of the name of a computer document
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。