file extension
Computing term for the suffix after a dot in a filename, indicating file type.
You can tell the type of file by looking at its extension.
拡張子を変更すると、ファイルが開けなくなることがあります。
Changing the file extension may make the file unopenable.
I changed the file's extension from .txt to .doc.
I changed the settings so that file extensions are shown for all files.
I checked the attached file's extension before opening it.