A file name has a portion called “extension.” The functions of extension can be summarized as follows.

• Generally, a file name can be divided into the portion normally called “file name” in a narrow meaning of file name, and the portion called “extension.”


• The narrow meaning of “file name” and “extension” are separated by a “.”(period).
  
    “exl.xcf”, “ex2.sxw”, “ex3.txt”
     In above file names, “ xcf,” “ sxw,” and “ txt” are the “extension.”


• In Windows systems, an “extension” usually consists of one to three characters.
  However, extensions that are commonly used for systems other than Windows systems, as in “html,” may carry four or more characters.


• “Extension” usually has some definite character set determined by the application program that creates the files.
  
    “sxw”← “StarSuite-Writer-”
    “txt” ← “Notepad”
    “xcf” ← “GIMP”
  Note:
  The files of software applications themselves generally have file names with extension “exe.”


• The icon of a file is determined by the “extension.” (More accurately, “extension” and file icon are associated.)
  Therefore, deleting the extension or writing incorrect extensions will lead to inappropriate icons. The files with such inappropriate icons may be inoperable, and you need to be careful.
  Note:
  Generally, however, attaching the wrong extension will not damage the file. When you notice an inappropriate extension, you can replace it with the correct one without hassle.

So far we have created images under GIMP, but there are a number of Painting software products other than GIMP. Although these are also Painting application software, they all save images in different file (data) formats, hence with different extensions. This means in general that a file created by an application program cannot be opened by another application program.

In the meantime, since image files generally have a very large data size, a number of saving formats have been devised to compress the data size while preserving as much image information as possible. For example, digital cameras and camera mounted on cell-phones as you may carry save the photos in the format called “JPEG,” whose extension is usually “jpg.” The saving format like this is a common format that does not depend on the application program.

Common image file formats and its extensions are shown below.

• “jpg” ← JPEG (Joint Photographic Experts Group) format
• “gif” ← GIF (Graphic Interchange Format) format
• “png” ← PNG (Portable Network Graphics) format

Painting application programs are designed to handle both image files in its own saving format, and image files of common saving formats.
In general, Painting application programs automatically determine the file format by the attached extension. You need to be careful for the extension, because using a wrong extension for saving will disable use of the file.

[Example of wrong operation 1]
Opening a file saved under sample.xcf, by the GIMP dedicated saving format, was attempted by StarSuite, only to create an error and fail.
(Causes and solutions)
A file with the GIMP dedicated saving format cannot be directly utilized by any other application. Save the file with a common format, such as JPEG, to enable reading by StarSuite.
To save by JPEG format under GIMP, select “Save by other name” upon saving, and select “JPEG” from “Saving option,” to attach the extension “jpg.”

[Example of wrong operation 2]
You saved a file under file name sample.xcf using the GIMP dedicated saving format, but then attempted to modify only the extension to carry “jpg.” Since this operation changed the icon that allowed reading by StarSuite, you attempted to read it using StarSuite, only to create an error without success.
(Causes and solutions)
Merely modifying the extension cannot change the file format of a file that has been saved once.
When saving a file under an application program such as GIMP, you need to change the file format you wish to choose.