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File Extensions explained




File Extensions explained

Every file on your computer has to follow a simple format, this includes the rootname, or base filename, and the file extension. In windows the filename and extension are separated by a period or a dot. By default, Windows 8, 7, Vista and XP, will hide file extensions of known file types.


Ex. Rootname.extension.  Examples of filename extensions are .png, .jpeg, .exe, .dmg and .txt.

The file extension generally tells the operating system how to read the data that is stored in the file. The file extension also defines which program will open the file by default. Before Windows 95 file extensions were limited to 3 characters. While this limit has been done away with since Windows 95 many programs maintain the 3 character extension to be backwards compatible.


Not all operating systems are the same

When transferring files between operating systems you can run into problems with the file names or extensions. UNIX, for example, uses a case sensitive file system so Image.extension and image.extension can be two different files. While Microsoft is case insensitive and will read Image.extension and image.extension as the same and will not allow them to be stored in the same directory.


Extensions can be spoofed

The user can manually change the file extension. For example I can change my word file (.doc) to a picture file (.jpg). Most of the time this will just cause an error and the program associated with that extension will not be able to read the file. However, you need to be careful. Sometimes a virus can be spoofed as a common file type (ie, .doc) and when you open it the virus will be initiated and spread. Many programs and virus scans today detect malicious files.


Filenames can have multiples periods and other symbols

A filename can have more than one period. For example, “Vacation 3.6.05-1.jpg” or “filename.txt.jpg.” The extension followed by the last period is used. Both of the examples will be read as a .jpg picture file by Windows.


The following are legal and illegal characters in a filename (the space became legal with Windows 95):

  • Legal: A-Z 0-9 $#&+@!()-{}'`_~, and the space
  • Illegal: |<>\^=?/[]";* plus control characters


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