A file extension is the short part at the end of a file name, after the period. In expenses.xlsx, the base name is expenses and the extension is xlsx. Windows identifies file types by their extension, and it uses the extension to choose the app and the icon for a file.
The File Type Extension is a field in a metadata report. It usually shows the extension for the kind of data inside the file, which can differ from the extension in the file name. The metadata reader ExifTool calls this field FileTypeExtension and describes it as a common lowercase extension for the file type.
What the value looks like
The value is a lowercase extension without the period, and it is usually 3 or 4 characters long. One file type can have several common extensions, but the field shows only one of them.
For example, a JPEG image can end in .jpeg, .jpg, or .jpe, and its File Type Extension is jpg. ExifTool gives these values for some common formats:
jpgfor a JPEG imagetiffor a TIFF imagepngfor a PNG imagemp3for an MP3 audio filempgfor an MPEG videopdffor a PDF document
Where the value comes from
The file does not store its File Type Extension. The metadata reader works out the value each time it reads the file. ExifTool lists the field as not writable, so you cannot edit it.
To find the type, ExifTool reads the first bytes of the file, called the header. A format can have a magic number, which is a fixed sequence of bytes at a set place in the file. ExifTool first tests the format that the extension in the file name suggests.
When a format has a magic number, the bytes must match it, whatever the file name says. The extension can still help decide the type, for example between camera raw formats built on TIFF, such as NEF and ARW. ExifTool then gives the usual extension for the type it found.
How the extension differs from the file type
The file type is the format of the data inside the file, such as JPEG or MP3. The extension is only part of the name, and anyone can change it when renaming a file.
A new extension does not convert the file to another format, because it changes nothing except the name. The file may then no longer open in the app that created it.
This is why a metadata report shows both values. The file name shows the extension the file was given, and the File Type Extension shows the extension that fits the content. The MIME type names the same file type in a third form.
Why it matters for your file
Compare the extension in the file name with the File Type Extension. If they match, the name usually fits the content. A JPEG image named photo.jpeg also fits, because jpeg and jpg are both common extensions for JPEG.
If they differ, check whether both are common extensions for the same format, such as tiff and tif. If they are not, the name may not fit the content. Your computer chooses the app by the extension, so the file can open in the wrong app or not open at all.
To compare the 2 values, you need to see the extension in the file name. Macs usually hide extensions. This is how you show them:
- On a Mac: in Finder, choose Settings from the Finder menu, click Advanced, and select Show all filename extensions
- In Windows 11: in File Explorer, select View, then Show, then File name extensions