What is the difference between a file extension and a file type?

The The terms "file extension" and "file type" are sometimes used interchangeably, but the two terms do have quite separate and distinct meanings.

A file extension is part of the file name. The extension is the last part of the file name after the dot. So for a file called "read-me.pdf" the file extension is ".pdf".

The file extension is used to determine the file type. So if you were to look up the file extension ".pdf" you would see that it has two file types associated with it. A ".pdf" file is either a portable document file or a printer description file. So in this example we have one file extension but two possible file types. Given in this example the file is called "read-me.pdf" the "read-me" part implies that it is something we need to read and thus its reasonable to conclude that it is a portable document file (PDF).

Frequently there is only one file type for a given file extension. Even when there are multiple possibilities there is often one file type which is the most frequent - take the example above "read-me.pdf", PDF files (portable document format) is a very widely used format which most people have come across, but the alternative of a printer description file is in contrast very rare indeed. So the file type often leads to a single file type. For this reason the terms file type and file extension are sometimes used interchangeably - although it is not always reasaonable to do so.

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