Learn in the textbookFile System Basics — Directory Management and Backup
DefinitionA file extension (ファイル拡張子) is the string after the last period in a file name that shows the file's type or format.
ExplanationThe OS uses the extension to decide which application opens the file and which icon to show. Because the extension is only part of the name, changing it does not convert the format of the data inside. When you search by replacing part of a file name with a symbol that stands in for any characters, that symbol is called a wildcard (ワイルドカード).
AnalogyIt is like writing "Invoice enclosed" on the front of an envelope. Anyone can tell at a glance what is inside, but rewriting those words does not change the document in the envelope.
Memory hookTo "extend" something is to make it longer. Think of the extension as the bit that lengthens the file name at the end.